PERF: removed coercion to int64 for arrays of ints in Categorical.from_codes
- [x] closes #18501
- [ ] tests added / **passed**
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
```
In [3]: arr = np.ones(10000000,dtype='int8')

# master
In [4]: %timeit pd.Categorical.from_codes(arr, ['foo', 'bar'])
44.2 ms ± 545 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)

# after patch
In [4]: %timeit pd.Categorical.from_codes(arr, ['foo', 'bar'])
9 ms ± 54.2 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
```

```
       before           after         ratio
     [6d5d7015]       [fb1f7b84]
         9.24±1ms       9.94±0.6ms     1.08  categoricals.Concat.time_concat
       5.52±0.1ms      5.41±0.05ms     0.98  categoricals.Concat.time_union
       32.0±0.3ms       32.3±0.3ms     1.01  categoricals.Constructor.time_all_nan
      1.32±0.02ms      1.28±0.01ms     0.97  categoricals.Constructor.time_datetimes
      1.26±0.01ms      1.29±0.02ms     1.02  categoricals.Constructor.time_datetimes_with_nat
          354±3μs          349±7μs     0.99  categoricals.Constructor.time_fastpath
      20.0±0.08ms       20.1±0.3ms     1.01  categoricals.Constructor.time_regular
          185±1ms        186±0.6ms     1.01  categoricals.Constructor.time_with_nan
           10.1ms           10.1ms     0.99  categoricals.Isin.time_isin_categorical('int64')
      10.7±0.08ms      10.8±0.07ms     1.00  categoricals.Isin.time_isin_categorical('object')
       9.11±0.1ms       9.04±0.2ms     0.99  categoricals.Rank.time_rank_int
       9.33±0.1ms       9.37±0.1ms     1.00  categoricals.Rank.time_rank_int_cat
       9.13±0.1ms      8.97±0.05ms     0.98  categoricals.Rank.time_rank_int_cat_ordered
        141±0.9ms          136±1ms     0.97  categoricals.Rank.time_rank_string
       11.2±0.2ms       11.1±0.1ms     0.99  categoricals.Rank.time_rank_string_cat
       9.04±0.1ms       9.23±0.1ms     1.02  categoricals.Rank.time_rank_string_cat_ordered
          592±5μs          586±3μs     0.99  categoricals.Repr.time_rendering
         32.8±2ms       28.4±0.6ms    ~0.86  categoricals.SetCategories.time_set_categories
         31.8±2ms       29.6±0.1ms     0.93  categoricals.ValueCounts.time_value_counts(False)
       30.7±0.1ms       29.3±0.2ms     0.96  categoricals.ValueCounts.time_value_counts(True)
```






Could you also add a release note (performance improvement)? # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21000?src=pr&el=h1) Report
> Merging [#21000](https://codecov.io/gh/pandas-dev/pandas/pull/21000?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/3d03fdb24b1a9c1438fe65f5242cbea5418787bf?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21000/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21000?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21000      +/-   ##
==========================================
- Coverage   91.82%   91.82%   -0.01%     
==========================================
  Files         153      153              
  Lines       49505    49502       -3     
==========================================
- Hits        45460    45457       -3     
  Misses       4045     4045
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.22% <100%> (-0.01%)` | :arrow_down: |
| #single | `41.88% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21000?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/21000/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `95.67% <100%> (ø)` | :arrow_up: |
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/21000/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `99.17% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/21000/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.64% <0%> (-0.01%)` | :arrow_down: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/21000/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.46% <0%> (ø)` | :arrow_up: |
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/21000/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `95.59% <0%> (ø)` | :arrow_up: |
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/21000/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvd2luZG93LnB5) | `96.28% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21000?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/21000?src=pr&el=footer). Last update [3d03fdb...75da8a3](https://codecov.io/gh/pandas-dev/pandas/pull/21000?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Updated ASV:
```
       before           after         ratio
     [3d03fdb2]       [d83abca4]
       9.27±0.6ms       9.13±0.3ms     0.98  categoricals.Concat.time_concat
       5.80±0.1ms      6.22±0.03ms     1.07  categoricals.Concat.time_union
       32.7±0.6ms       32.8±0.4ms     1.00  categoricals.Constructor.time_all_nan
      1.26±0.03ms      1.24±0.01ms     0.99  categoricals.Constructor.time_datetimes
         1.28±0ms      1.28±0.01ms     1.00  categoricals.Constructor.time_datetimes_with_nat
          364±8μs          348±5μs     0.96  categoricals.Constructor.time_fastpath
      19.9±0.08ms      20.0±0.06ms     1.00  categoricals.Constructor.time_regular
        181±0.5ms          182±2ms     1.00  categoricals.Constructor.time_with_nan
           10.2ms           10.3ms     1.01  categoricals.Isin.time_isin_categorical('int64')
       10.8±0.1ms      10.5±0.09ms     0.97  categoricals.Isin.time_isin_categorical('object')
      8.89±0.08ms       8.94±0.1ms     1.01  categoricals.Rank.time_rank_int
       9.33±0.2ms      9.44±0.05ms     1.01  categoricals.Rank.time_rank_int_cat
      8.95±0.08ms      8.95±0.05ms     1.00  categoricals.Rank.time_rank_int_cat_ordered
          135±2ms          140±2ms     1.03  categoricals.Rank.time_rank_string
      11.0±0.06ms       11.1±0.2ms     1.01  categoricals.Rank.time_rank_string_cat
       9.14±0.2ms       9.11±0.1ms     1.00  categoricals.Rank.time_rank_string_cat_ordered
          590±7μs         599±20μs     1.01  categoricals.Repr.time_rendering
       27.7±0.5ms       29.1±0.2ms     1.05  categoricals.SetCategories.time_set_categories
       30.0±0.4ms       29.6±0.4ms     0.99  categoricals.ValueCounts.time_value_counts(False)
       29.6±0.4ms       29.6±0.2ms     1.00  categoricals.ValueCounts.time_value_counts(True)
```

 I think @jbreack was requesting a *new* ASV that measures `Categorical.from_codes` with an `int8`  type array.

It can go in `asv_bench/benchmarks/categoricals.py` under `Constructor`. Sorry, I misunderstood. Thanks for the clarification. Is the following asv sufficient?

```
diff --git a/asv_bench/benchmarks/categoricals.py b/asv_bench/benchmarks/categoricals.py
index 0ffd5f881..ae1d70292 100644
--- a/asv_bench/benchmarks/categoricals.py
+++ b/asv_bench/benchmarks/categoricals.py
@@ -51,6 +51,7 @@ class Constructor(object):

         self.values_some_nan = list(np.tile(self.categories + [np.nan], N))
         self.values_all_nan = [np.nan] * len(self.values)
+        self.values_all_int8 = np.ones(N, 'int8')

     def time_regular(self):
         pd.Categorical(self.values, self.categories)
@@ -70,6 +71,9 @@ class Constructor(object):
     def time_all_nan(self):
         pd.Categorical(self.values_all_nan)

+    def time_from_codes_all_int8(self):
+        pd.Categorical.from_codes(self.values_all_int8, self.categories)
+
```