BUG: Handle IntegerArray in pd.cut
xref https://github.com/pandas-dev/pandas/issues/30944.
I think this doesn't close it, since only the pd.cut compoment
is fixed.

cc @jorisvandenbossche @jreback. The changes here attempt to be extremely conservative, since we're backporting stuff. I'm trying to not change behavior for anything other than IntegerArray. In particular, I'm not trying to support arbitrary EAs in `pd.cut`. This leads to some code that's fairly ugly & specific to IntegerArray. I think we should attempt to clean that up in 1.1.