CLN/PERF: clean-up of the benchmarks
Related to https://github.com/pydata/pandas/issues/10849 (similar to https://github.com/pydata/pandas/pull/10998)

Just putting this up, was doing some time ago, but not yet fully ready, and thought to already merge this (if the content is OK) to not get too much conflicts.

Gist: removing redundancy by gathering tests in classes and sharing setup functions (you can see the diff change that it removed quite some lines of code). At the same time, I am cleaning up the test names a bit, to make them shorter/more pythonic (eg `DatetimeIndex.time_add_timedelta` instead of `time_datetimeindex_add_timedelta.time_datetimeindex_add_timedelta`). 
The only disadvantage of this cleanup is that for people who already did run the benchmarks for older versions of pandas, this will have to be redone.

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14099?src=pr) is 85.26% (diff: 100%)
> Merging [#14099](https://codecov.io/gh/pandas-dev/pandas/pull/14099?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

```diff
@@             master     #14099   diff @@
==========================================
  Files           144        140     -4   
  Lines         50979      50667   -312   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43468      43201   -267   
+ Misses         7511       7466    -45   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [1dbc7be...ab32803](https://codecov.io/gh/pandas-dev/pandas/compare/1dbc7bedcde3274d945790d17c864eecf787a9fa...ab328031ed3241c0be401780ec71a523b9140663?src=pr) xref #14105: Make sure to remove all references to `datetools` both in `vb_suites` and `asv_bench`.
 @jorisvandenbossche skimmed lgtm. Though I didn't run this, so assuming correctness, go ahead.
 @jorisvandenbossche merge when ready.
 Will try to clean the remaining ones next week
 let's merge this
 side issue, we should prob pep8 the asv code :> (and of course include it in our tests)
 should rebase and merge Todo' from comments:

- deal with the new resample API (see https://github.com/pandas-dev/pandas/pull/14099#r76518712)
- consolidate the benchmarks for the io functions (as you now have `parsers_vb.py`, but also `io_bench.py`, `hdfstore_bench.py`, `io_sql.py`). Rename parser_vb -> parser
- further clean-up
- better structure? (subdirs?)
- PEP8