ExtensionArray meta-issue
Just so things don't get lost

- [x] Interface, ExtensionBlock, and Internals (#19268)
- [x] Series and DataFrame can hold extension arrays (#19520)
- [x] overridable `assert_*_equal` (https://github.com/pandas-dev/pandas/pull/19863)
- [x] unique (#19869)
- [x] fillna (#19909)
- [x] `__setitem__` works properly (https://github.com/pandas-dev/pandas/pull/19907)
- [x] Document everything: (#19936)
- [x] argsort, sort_values (https://github.com/pandas-dev/pandas/pull/19957)
- [x] factorize (https://github.com/pandas-dev/pandas/pull/20361)
- [x] groupby works properly (blocked by factorize)
- [x] IntervalArray: (https://github.com/TomAugspurger/pandas/tree/pandas-array-upstream%2Bfu1%2Binterval)
- [x] PeriodArray: index backed by PerioidArray, store in Series / Frame
- [x] Refactor arithmetic methods from DatetimeIndexOpsMixin/DatetimeIndex/TimedeltaIndex/PeriodIndex into array classes (#19902)
- [x] Refactor comparison methods from DTI/TDI/PI into array classes
- [ ] select_dtypes