CLN: split off frozen (immutable) data structures into pandas/indexes/frozen.py
should make it a bit easier to work with these; and no reason to be in pandas/core/base.py 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15477?src=pr&el=h1) Report
> Merging [#15477](https://codecov.io/gh/pandas-dev/pandas/pull/15477?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b94186d4c58ee055656a84f55618be537db0095a?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `86.07%`.


```diff
@@            Coverage Diff             @@
##           master   #15477      +/-   ##
==========================================
+ Coverage   90.36%   90.36%   +<.01%     
==========================================
  Files         135      136       +1     
  Lines       49519    49532      +13     
==========================================
+ Hits        44747    44759      +12     
- Misses       4772     4773       +1
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15477?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/compare/b94186d4c58ee055656a84f55618be537db0095a...2a64a4f74cb9d5014e0d424cd23dbee26efefaeb?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `95.51% <ø> (+0.35%)` | :white_check_mark: |
| [pandas/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/compare/b94186d4c58ee055656a84f55618be537db0095a...2a64a4f74cb9d5014e0d424cd23dbee26efefaeb?src=pr&el=tree#diff-cGFuZGFzL2luZGV4ZXMvbXVsdGkucHk=) | `96.65% <100%> (ø)` | :white_check_mark: |
| [pandas/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/compare/b94186d4c58ee055656a84f55618be537db0095a...2a64a4f74cb9d5014e0d424cd23dbee26efefaeb?src=pr&el=tree#diff-cGFuZGFzL2luZGV4ZXMvYmFzZS5weQ==) | `96.22% <100%> (-0.02%)` | :x: |
| [pandas/compat/pickle_compat.py](https://codecov.io/gh/pandas-dev/pandas/compare/b94186d4c58ee055656a84f55618be537db0095a...2a64a4f74cb9d5014e0d424cd23dbee26efefaeb?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9waWNrbGVfY29tcGF0LnB5) | `68.29% <41.66%> (+4.4%)` | :white_check_mark: |
| [pandas/indexes/frozen.py](https://codecov.io/gh/pandas-dev/pandas/compare/b94186d4c58ee055656a84f55618be537db0095a...2a64a4f74cb9d5014e0d424cd23dbee26efefaeb?src=pr&el=tree#diff-cGFuZGFzL2luZGV4ZXMvZnJvemVuLnB5) | `93.54% <93.54%> (ø)` | |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/compare/b94186d4c58ee055656a84f55618be537db0095a...2a64a4f74cb9d5014e0d424cd23dbee26efefaeb?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.02% <ø> (-0.34%)` | :x: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15477?src=pr&el=footer). Last update [b94186d...2a64a4f](https://codecov.io/gh/pandas-dev/pandas/compare/b94186d4c58ee055656a84f55618be537db0095a...2a64a4f74cb9d5014e0d424cd23dbee26efefaeb?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).