CategoricalDtype follows after #16015

[jreback]

• use display.max_categories in CagetegoricalDtype.__unicode__ here
• use display.max_categories in Categorical.__unicode__
• deprecate usage of fastpath in Categorical constructor (API: Deprecate Categorical fastpath argument #17562)
• deprecate .astype(..., categories=) in favor of CategoricalDtype (DEPR: .astype(..., categories=) #17636)
• move hash type routines to hashing.py here
• validate that pyarrow is compat with changes

[jreback]

@TomAugspurger

[jreback]

cc @wesm

I don't think pyarrow will be impacted by this; it is possible that knowing the categories upfront could be helpful though

[wesm]

I don't think it would be, but we should check anyway and see if there's any perf improvement opportunities

[mroeschke]

Looks like most of the associated issues are closed. Can open specific issues if there are specifics that were missed