PERF: avoid unneeded recoding of categoricals and reuse CategoricalDtypes for greater slicing speed
- [x] progress towards #20395
- [x] ASVs added
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Currently, a lot of indexing/slicing ops on CategoricalIndexes goes through ``CategoricalIndex._create_categorical``, which can be a slow operation, because calling  ``data._set_dtype`` there is slow. This PR improves performance by avoiding calling ``data._set_dtype`` as often, specifically when the new and the old dtypes are equal.

An complicating issue was that comparisons of ``CategoricalDtype`` was quite slow, so the improvements I saw in #20395 were offset by slowdowns in other places. To avoid this, ``CategoricalDtype.__equal__`` needed to become smarter and pandas has to pass round existing dtypes rather than only ``categories`` and ``ordered``. This is ok, as CategoricalDtype is immutable. This minimizes the need to call the expensive ``CategoricalDtype.__hash__`` method in ``CategoricalDtype.__equal__`` and makes comparisons much faster.

### Some notable results

First setup:

```python
>>> n = 100_000
>>> c = pd.Categorical(list('a' * n + 'b' * n + 'c' * n))
>>> ci = pd.CategoricalIndex(c)
>>> df = pd.DataFrame({'A': range(n * 3)}, index=ci)
>>> sl = slice(n, n * 2)
```

Results:

```python
>>> %timeit c[sl]
13.9 µs  # master
4.43 µs  # this PR
>>> %timeit ci[sl]
740 µs  # master
12.7 µs  # this PR
>>> %timeit df.iloc[sl]
855 µs  # master
72.2 µs  # this PR
>>> %timeit df.loc['b']
3.23 ms  # master
1.62 ms  # this PR
```

### Benchmarks

benchmarks/indexing.py:

```
      before           after         ratio
     [36422a88]       [e0c62df0]
+        53.4±0μs         61.1±0μs     1.14  indexing.NumericSeriesIndexing.time_iloc_slice(<class 'pandas.core.indexes.numeric.Int64Index'>)
-         477±4ms          414±4ms     0.87  indexing.CategoricalIndexIndexing.time_get_indexer_list('monotonic_incr')
-        476±40ns         381±20ns     0.80  indexing.MethodLookup.time_lookup_iloc
-      1.23±0.2ms          367±0μs     0.30  indexing.CategoricalIndexIndexing.time_getitem_bool_array('monotonic_decr')
-      1.29±0.2ms          344±5μs     0.27  indexing.CategoricalIndexIndexing.time_getitem_bool_array('monotonic_incr')
-         115±8μs         19.5±2μs     0.17  indexing.CategoricalIndexIndexing.time_getitem_list_like('monotonic_decr')
-         122±2μs         19.5±0μs     0.16  indexing.CategoricalIndexIndexing.time_getitem_list_like('non_monotonic')
-         125±8μs         19.8±2μs     0.16  indexing.CategoricalIndexIndexing.time_getitem_list_like('monotonic_incr')
-         121±2μs         13.4±0μs     0.11  indexing.CategoricalIndexIndexing.time_getitem_slice('non_monotonic')
-         129±2μs         12.5±0μs     0.10  indexing.CategoricalIndexIndexing.time_getitem_slice('monotonic_incr')
-        195±20μs       13.4±0.2μs     0.07  indexing.CategoricalIndexIndexing.time_getitem_slice('monotonic_decr')

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
```

benchmarks/categoricals.py:

```
      before           after         ratio
     [a620e725]       [04667f67]
+        10.4±0ms       11.7±0.5ms     1.12  categoricals.Concat.time_union
+          3.42μs           3.76μs     1.10  categoricals.CategoricalSlicing.time_getitem_scalar('non_monotonic')
-          3.66μs           3.20μs     0.87  categoricals.CategoricalSlicing.time_getitem_scalar('monotonic_incr')
-      20.8±0.7ms         13.9±0ms     0.67  categoricals.ValueCounts.time_value_counts(False)
-        23.4±0ms         12.2±0ms     0.52  categoricals.ValueCounts.time_value_counts(True)
-          13.3μs           5.86μs     0.44  categoricals.CategoricalSlicing.time_getitem_slice('monotonic_decr')
-          13.3μs           4.88μs     0.37  categoricals.CategoricalSlicing.time_getitem_slice('non_monotonic')
-          17.1μs           6.12μs     0.36  categoricals.CategoricalSlicing.time_getitem_list_like('monotonic_incr')
-          17.1μs           6.11μs     0.36  categoricals.CategoricalSlicing.time_getitem_list_like('non_monotonic')
-          15.5μs           4.39μs     0.28  categoricals.CategoricalSlicing.time_getitem_slice('monotonic_incr')
-          22.0μs           6.11μs     0.28  categoricals.CategoricalSlicing.time_getitem_list_like('monotonic_decr')

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
```

I haven't run the whole test suite, as that takes a long time (4-5 hours?) for me. Would appreciate input first and, if I need to run the whole suite, if there is a smarter way to do it.
The CircleCi error seems unrelated : ``AssertionError: Caused unexpected warning(s): ['ResourceWarning'].`` in pandas/tests/io/parser/dialect.py, which isn't part of this PR. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21659?src=pr&el=h1) Report
> Merging [#21659](https://codecov.io/gh/pandas-dev/pandas/pull/21659?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a620e7255ce896c9185275d82172513268b1a719?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21659/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21659?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21659      +/-   ##
==========================================
+ Coverage    91.9%    91.9%   +<.01%     
==========================================
  Files         154      154              
  Lines       49555    49559       +4     
==========================================
+ Hits        45542    45546       +4     
  Misses       4013     4013
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.27% <100%> (ø)` | :arrow_up: |
| #single | `42.02% <100%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21659?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/21659/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `95.63% <ø> (ø)` | :arrow_up: |
| [pandas/core/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/pull/21659/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9jYXRlZ29yeS5weQ==) | `97.01% <100%> (ø)` | :arrow_up: |
| [pandas/core/dtypes/dtypes.py](https://codecov.io/gh/pandas-dev/pandas/pull/21659/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2R0eXBlcy5weQ==) | `95.95% <100%> (+0.04%)` | :arrow_up: |

------

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> **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/21659?src=pr&el=footer). Last update [a620e72...ff2de3f](https://codecov.io/gh/pandas-dev/pandas/pull/21659?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Comments accounted for. thanks @topper-123 