API: Uses pd.NA in IntegerArray
Just a WIP for now. I haven't closely reviewed this since pd.NA was merged into master.

cc @jorisvandenbossche 

Closes https://github.com/pandas-dev/pandas/issues/15375
Still some failing tests. Will have to put this on hold for a day or two now. The newly added tests from the other PR are failing (they still assume bool numpy output) ```
pandas/tests/test_register_accessor.py:39: error: Module has no attribute "register_series_accessor"
pandas/tests/test_register_accessor.py:40: error: Module has no attribute "register_dataframe_accessor"
pandas/tests/test_register_accessor.py:41: error: Module has no attribute "register_index_accessor"
```

I'm not able to reproduce these locally. What's the status of this? This is ready to go? (with the special cases for pow etc already merged in other PRs) > What's the status of this? This is ready to go? (with the special cases for pow etc already merged in other PRs)

I think it's ready. All the `__pow__` stuff was handled elsewhere. The only pow-related changes were to add tests with `pd.NA` like https://github.com/pandas-dev/pandas/pull/29964/files#diff-3523d268da9f1c7d4c82d4d27241ed07R373. I am planning to merge this in a bit. I am not sure if @jreback's review was a full one, but the comments have been addressed/answered. But it would be good to have this in master for IntegerArray as well, and this is blocking some follow-ups (completing `to_numpy` in #30322, creating the base class). There can be more review in those PRs I think. let me have a look a n hour or 2 @jreback did you still want to / have time to take a look? let me look Thanks, added whatsnew for those changes. All green. Anything else @jreback or were the two in https://github.com/pandas-dev/pandas/pull/29964#pullrequestreview-337098618 it? thanks @TomAugspurger 