CLN: ASV io_bench, parser_vb
There are a lot of io benchmarks scattered among `hdfstore_bench.py`, `io_bench.py`, `parser_vb.py`, and `packer.py`. I think it would be a lot cleaner if each io method had it's own file (like `io_sql.py` which already exists)

This PR creates `io_csv.py` and `io_json.py` that consolidates the csv and json benchmarks that exist. Benchmarks were flake8'd, `param`'d, and simplified where available:

```
$ asv dev -b ^io_json
· Discovering benchmarks
· Running 15 total benchmarks (1 commits * 1 environments * 15 benchmarks)
[  0.00%] ·· Building for existing-py_home_matt_anaconda_envs_pandas_dev_bin_python
[  0.00%] ·· Benchmarking existing-py_home_matt_anaconda_envs_pandas_dev_bin_python
[  6.67%] ··· Running io_json.ReadJSON.time_read_json                                                              ok
[  6.67%] ···· 
               ========= ======= ==========
               --              index       
               --------- ------------------
                 orient    int    datetime 
               ========= ======= ==========
                 split    294ms    272ms   
                 index    7.61s    7.65s   
                records   614ms    613ms   
               ========= ======= ==========

[ 13.33%] ··· Running io_json.ReadJSONLines.peakmem_read_json_lines                                                ok
[ 13.33%] ···· 
               ========== ======
                 index          
               ---------- ------
                  int      192M 
                datetime   192M 
               ========== ======

[ 20.00%] ··· Running io_json.ReadJSONLines.peakmem_read_json_lines_concat                                         ok
[ 20.00%] ···· 
               ========== ======
                 index          
               ---------- ------
                  int      164M 
                datetime   164M 
               ========== ======

[ 26.67%] ··· Running io_json.ReadJSONLines.time_read_json_lines                                                   ok
[ 26.67%] ···· 
               ========== =======
                 index           
               ---------- -------
                  int      755ms 
                datetime   720ms 
               ========== =======

[ 33.33%] ··· Running io_json.ReadJSONLines.time_read_json_lines_concat                                            ok
[ 33.33%] ···· 
               ========== =======
                 index           
               ---------- -------
                  int      749ms 
                datetime   752ms 
               ========== =======

[ 40.00%] ··· Running io_json.ToJSON.time_delta_int_tstamp                                                         ok
[ 40.00%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    238ms 
                columns   234ms 
                 index    397ms 
               ========= =======

[ 46.67%] ··· Running io_json.ToJSON.time_delta_int_tstamp_lines                                                   ok
[ 46.67%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    593ms 
                columns   566ms 
                 index    540ms 
               ========= =======

[ 53.33%] ··· Running io_json.ToJSON.time_float_int                                                                ok
[ 53.33%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    237ms 
                columns   214ms 
                 index    371ms 
               ========= =======

[ 60.00%] ··· Running io_json.ToJSON.time_float_int_lines                                                          ok
[ 60.00%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    569ms 
                columns   570ms 
                 index    570ms 
               ========= =======

[ 66.67%] ··· Running io_json.ToJSON.time_float_int_str                                                            ok
[ 66.67%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    238ms 
                columns   216ms 
                 index    384ms 
               ========= =======

[ 73.33%] ··· Running io_json.ToJSON.time_float_int_str_lines                                                      ok
[ 73.33%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    598ms 
                columns   614ms 
                 index    617ms 
               ========= =======

[ 80.00%] ··· Running io_json.ToJSON.time_floats_with_dt_index                                                     ok
[ 80.00%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    182ms 
                columns   209ms 
                 index    208ms 
               ========= =======

[ 86.67%] ··· Running io_json.ToJSON.time_floats_with_dt_index_lines                                               ok
[ 86.67%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    442ms 
                columns   444ms 
                 index    449ms 
               ========= =======

[ 93.33%] ··· Running io_json.ToJSON.time_floats_with_int_idex_lines                                               ok
[ 93.33%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    442ms 
                columns   442ms 
                 index    441ms 
               ========= =======

[100.00%] ··· Running io_json.ToJSON.time_floats_with_int_index                                                    ok
[100.00%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    159ms 
                columns   171ms 
                 index    182ms 
               ========= =======
```

```
asv dev -b ^io_csv
· Discovering benchmarks
· Running 16 total benchmarks (1 commits * 1 environments * 16 benchmarks)
[  0.00%] ·· Building for existing-py_home_matt_anaconda_envs_pandas_dev_bin_python
[  0.00%] ·· Benchmarking existing-py_home_matt_anaconda_envs_pandas_dev_bin_python
[  6.25%] ··· Running io_csv.ReadCSVCategorical.time_convert_direct                                            99.1ms
[ 12.50%] ··· Running io_csv.ReadCSVCategorical.time_convert_post                                               139ms
[ 18.75%] ··· Running io_csv.ReadCSVComment.time_comment                                                       60.7ms
[ 25.00%] ··· Running io_csv.ReadCSVDInferDatetimeFormat.time_read_csv                                             ok
[ 25.00%] ···· 
               ======================= ======== ========= ========
               --                                 format          
               ----------------------- ---------------------------
                infer_datetime_format   custom   iso8601    ymd   
               ======================= ======== ========= ========
                         True           23.0ms    4.65ms   4.77ms 
                        False           619ms     3.49ms   3.18ms 
               ======================= ======== ========= ========

[ 31.25%] ··· Running io_csv.ReadCSVFloatPrecision.time_read_csv                                                   ok
[ 31.25%] ···· 
               ===== ========== ========== ================ ========== ========== ================
               --                              decimal / float_precision                          
               ----- -----------------------------------------------------------------------------
                sep   . / None   . / high   . / round_trip   _ / None   _ / high   _ / round_trip 
               ===== ========== ========== ================ ========== ========== ================
                 ,     3.89ms     3.73ms        5.12ms        4.17ms     4.10ms        4.16ms     
                 ;     3.87ms     3.71ms        5.23ms        4.11ms     4.18ms        4.23ms     
               ===== ========== ========== ================ ========== ========== ================

[ 37.50%] ··· Running io_csv.ReadCSVFloatPrecision.time_read_csv_python_engine                                     ok
[ 37.50%] ···· 
               ===== ========== ========== ================ ========== ========== ================
               --                              decimal / float_precision                          
               ----- -----------------------------------------------------------------------------
                sep   . / None   . / high   . / round_trip   _ / None   _ / high   _ / round_trip 
               ===== ========== ========== ================ ========== ========== ================
                 ,     7.84ms     7.76ms        7.80ms        6.14ms     6.24ms        6.25ms     
                 ;     7.76ms     7.80ms        8.41ms        6.10ms     6.25ms        6.14ms     
               ===== ========== ========== ================ ========== ========== ================

[ 43.75%] ··· Running io_csv.ReadCSVParseDates.time_baseline                                                   2.85ms
[ 50.00%] ··· Running io_csv.ReadCSVParseDates.time_multiple_date                                              2.84ms
[ 56.25%] ··· Running io_csv.ReadCSVSkipRows.time_skipprows                                                        ok
[ 56.25%] ···· 
               ========== ========
                skiprows          
               ---------- --------
                  None     44.8ms 
                 10000     30.9ms 
               ========== ========

[ 62.50%] ··· Running io_csv.ReadCSVThousands.time_thousands                                                       ok
[ 62.50%] ···· 
               ===== ======== ========
               --        thousands    
               ----- -----------------
                sep    None      ,    
               ===== ======== ========
                 ,    38.5ms   37.0ms 
                 |    37.8ms   39.6ms 
               ===== ======== ========

[ 68.75%] ··· Running io_csv.ReadUint64Integers.time_read_uint64                                               8.99ms
[ 75.00%] ··· Running io_csv.ReadUint64Integers.time_read_uint64_na_values                                     13.2ms
[ 81.25%] ··· Running io_csv.ReadUint64Integers.time_read_uint64_neg_values                                    12.9ms
[ 87.50%] ··· Running io_csv.S3.time_read_csv_10_rows                                                              ok
[ 87.50%] ···· 
               ============= ======== =======
               --                 engine     
               ------------- ----------------
                compression   python     c   
               ============= ======== =======
                    None      6.41s    6.13s 
                    gzip      6.20s    6.60s 
                    bz2       35.1s     n/a  
               ============= ======== =======

[ 93.75%] ··· Running io_csv.ToCSV.time_frame                                                                      ok
[ 93.75%] ···· 
               ======= ========
                 kind          
               ------- --------
                 wide   84.4ms 
                 long   164ms  
                mixed   37.4ms 
               ======= ========
```

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18815?src=pr&el=h1) Report
> Merging [#18815](https://codecov.io/gh/pandas-dev/pandas/pull/18815?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b6a7cc9d4686f62596de031c9cc4a1d62d12c52e?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18815/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18815?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18815      +/-   ##
==========================================
- Coverage   91.64%   91.62%   -0.02%     
==========================================
  Files         154      154              
  Lines       51408    51408              
==========================================
- Hits        47113    47104       -9     
- Misses       4295     4304       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.49% <ø> (ø)` | :arrow_up: |
| #single | `40.83% <ø> (-0.12%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18815?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18815/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18815/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.68% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18815?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18815?src=pr&el=footer). Last update [b6a7cc9...e8eda6a](https://codecov.io/gh/pandas-dev/pandas/pull/18815?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 +1 on this, does asv supported nested benchmarks, e.g. can we create a ``benchmarks/io/json.....`` and so on and have it work? Yup. Created a new `io` folder with `csv.py` and `json.py`

```
asv dev -b ^io[^_]
· Discovering benchmarks
· Running 31 total benchmarks (1 commits * 1 environments * 31 benchmarks)
[  0.00%] ·· Building for existing-py_home_matt_anaconda_envs_pandas_dev_bin_python
[  0.00%] ·· Benchmarking existing-py_home_matt_anaconda_envs_pandas_dev_bin_python
[  3.23%] ··· Running io.csv.ReadCSVCategorical.time_convert_direct                                          98.1ms
[  6.45%] ··· Running io.csv.ReadCSVCategorical.time_convert_post                                             143ms
[  9.68%] ··· Running io.csv.ReadCSVComment.time_comment                                                     55.6ms
[ 12.90%] ··· Running io.csv.ReadCSVDInferDatetimeFormat.time_read_csv                                           ok
[ 12.90%] ···· 
               ======================= ======== ========= ========
               --                                 format          
               ----------------------- ---------------------------
                infer_datetime_format   custom   iso8601    ymd   
               ======================= ======== ========= ========
                         True           22.9ms    4.62ms   4.83ms 
                        False           615ms     3.46ms   3.22ms 
               ======================= ======== ========= ========

[ 16.13%] ··· Running io.csv.ReadCSVFloatPrecision.time_read_csv                                                 ok
[ 16.13%] ···· 
               ===== ========== ========== ================ ========== ========== ================
               --                              decimal / float_precision                          
               ----- -----------------------------------------------------------------------------
                sep   . / None   . / high   . / round_trip   _ / None   _ / high   _ / round_trip 
               ===== ========== ========== ================ ========== ========== ================
                 ,     3.97ms     4.01ms        5.18ms        4.20ms     4.17ms        4.14ms     
                 ;     4.08ms     3.75ms        5.27ms        4.12ms     4.09ms        4.24ms     
               ===== ========== ========== ================ ========== ========== ================

[ 19.35%] ··· Running io.csv.ReadCSVFloatPrecision.time_read_csv_python_engine                                   ok
[ 19.35%] ···· 
               ===== ========== ========== ================ ========== ========== ================
               --                              decimal / float_precision                          
               ----- -----------------------------------------------------------------------------
                sep   . / None   . / high   . / round_trip   _ / None   _ / high   _ / round_trip 
               ===== ========== ========== ================ ========== ========== ================
                 ,     7.93ms     7.76ms        7.70ms        6.14ms     6.21ms        6.21ms     
                 ;     7.69ms     7.80ms        7.72ms        6.16ms     6.36ms        6.23ms     
               ===== ========== ========== ================ ========== ========== ================

[ 22.58%] ··· Running io.csv.ReadCSVParseDates.time_baseline                                                 2.91ms
[ 25.81%] ··· Running io.csv.ReadCSVParseDates.time_multiple_date                                            2.91ms
[ 29.03%] ··· Running io.csv.ReadCSVSkipRows.time_skipprows                                                      ok
[ 29.03%] ···· 
               ========== ========
                skiprows          
               ---------- --------
                  None     46.1ms 
                 10000     30.7ms 
               ========== ========

[ 32.26%] ··· Running io.csv.ReadCSVThousands.time_thousands                                                     ok
[ 32.26%] ···· 
               ===== ======== ========
               --        thousands    
               ----- -----------------
                sep    None      ,    
               ===== ======== ========
                 ,    38.3ms   35.9ms 
                 |    39.1ms   38.6ms 
               ===== ======== ========

[ 35.48%] ··· Running io.csv.ReadUint64Integers.time_read_uint64                                             8.76ms
[ 38.71%] ··· Running io.csv.ReadUint64Integers.time_read_uint64_na_values                                   13.1ms
[ 41.94%] ··· Running io.csv.ReadUint64Integers.time_read_uint64_neg_values                                  13.1ms
[ 45.16%] ··· Running io.csv.S3.time_read_csv_10_rows                                                            ok
[ 45.16%] ···· 
               ============= ======== =======
               --                 engine     
               ------------- ----------------
                compression   python     c   
               ============= ======== =======
                    None      6.15s    6.53s 
                    gzip      6.78s    6.15s 
                    bz2       34.0s     n/a  
               ============= ======== =======

[ 48.39%] ··· Running io.csv.ToCSV.time_frame                                                                    ok
[ 48.39%] ···· 
               ======= ========
                 kind          
               ------- --------
                 wide   107ms  
                 long   190ms  
                mixed   41.4ms 
               ======= ========

[ 51.61%] ··· Running io.csv.ToCSVDatetime.time_frame_date_formatting                                        21.9ms
[ 54.84%] ··· Running io.json.ReadJSON.time_read_json                                                            ok
[ 54.84%] ···· 
               ========= ======= ==========
               --              index       
               --------- ------------------
                 orient    int    datetime 
               ========= ======= ==========
                 split    254ms    270ms   
                 index    7.43s    7.55s   
                records   610ms    618ms   
               ========= ======= ==========

[ 58.06%] ··· Running io.json.ReadJSONLines.peakmem_read_json_lines                                              ok
[ 58.06%] ···· 
               ========== ======
                 index          
               ---------- ------
                  int      192M 
                datetime   192M 
               ========== ======

[ 61.29%] ··· Running io.json.ReadJSONLines.peakmem_read_json_lines_concat                                       ok
[ 61.29%] ···· 
               ========== ======
                 index          
               ---------- ------
                  int      164M 
                datetime   164M 
               ========== ======

[ 64.52%] ··· Running io.json.ReadJSONLines.time_read_json_lines                                                 ok
[ 64.52%] ···· 
               ========== =======
                 index           
               ---------- -------
                  int      715ms 
                datetime   711ms 
               ========== =======

[ 67.74%] ··· Running io.json.ReadJSONLines.time_read_json_lines_concat                                          ok
[ 67.74%] ···· 
               ========== =======
                 index           
               ---------- -------
                  int      748ms 
                datetime   748ms 
               ========== =======

[ 70.97%] ··· Running io.json.ToJSON.time_delta_int_tstamp                                                       ok
[ 70.97%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    236ms 
                columns   231ms 
                 index    381ms 
               ========= =======

[ 74.19%] ··· Running io.json.ToJSON.time_delta_int_tstamp_lines                                                 ok
[ 74.19%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    541ms 
                columns   541ms 
                 index    539ms 
               ========= =======

[ 77.42%] ··· Running io.json.ToJSON.time_float_int                                                              ok
[ 77.42%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    220ms 
                columns   213ms 
                 index    370ms 
               ========= =======

[ 80.65%] ··· Running io.json.ToJSON.time_float_int_lines                                                        ok
[ 80.65%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    577ms 
                columns   569ms 
                 index    567ms 
               ========= =======

[ 83.87%] ··· Running io.json.ToJSON.time_float_int_str                                                          ok
[ 83.87%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    237ms 
                columns   214ms 
                 index    387ms 
               ========= =======

[ 87.10%] ··· Running io.json.ToJSON.time_float_int_str_lines                                                    ok
[ 87.10%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    595ms 
                columns   598ms 
                 index    600ms 
               ========= =======

[ 90.32%] ··· Running io.json.ToJSON.time_floats_with_dt_index                                                   ok
[ 90.32%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    181ms 
                columns   206ms 
                 index    208ms 
               ========= =======

[ 93.55%] ··· Running io.json.ToJSON.time_floats_with_dt_index_lines                                             ok
[ 93.55%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    446ms 
                columns   447ms 
                 index    444ms 
               ========= =======

[ 96.77%] ··· Running io.json.ToJSON.time_floats_with_int_idex_lines                                             ok
[ 96.77%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    439ms 
                columns   439ms 
                 index    440ms 
               ========= =======

[100.00%] ··· Running io.json.ToJSON.time_floats_with_int_index                                                  ok
[100.00%] ···· 
               ========= =======
                 orient         
               --------- -------
                 split    160ms 
                columns   173ms 
                 index    183ms 
               ========= =======
``` thanks @mroeschke 

ideally would consolidate the rest of the io benchmarks in io/ 