ENH: Allow dict as the argument to Index.map (GH13517)
- [X] closes #13517
- [X] tests added / passed
- [X] passes `git diff upstream/master | flake8 --diff`
- [X] whatsnew entry

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13518?src=pr) is **84.33%**

> Merging [#13518](https://codecov.io/gh/pydata/pandas/pull/13518?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13518   diff @@
==========================================
  Files           138        138          
  Lines         51107      51110     +3   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43103      43106     +3   
  Misses         8004       8004          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [bf4786a...0b32709](https://codecov.io/gh/pydata/pandas/compare/bf4786a15f51d14ccf1318e44ae080868d8595ee...0b327096830e54b741f15737e344fb0fdf67aac9)
 I would ideally like to merge the `Series.map` and `Index.map` impl and move it to `Base.map` like most other methods. You can move tests to `tests/test_base/py:TestIndexOps`. This will make these more consistent.
 can you rebase / update?
 closing, but pls reopen / comment if you can continue
