[WIP]: API ExtensionDtype for DTA & TDA
Closes https://github.com/pandas-dev/pandas/issues/24662

TODO:

- [ ] Dedicated tests for new is_<foo> types
- [ ] Run the extension dtype tests for new dtypes
- [ ] (maybe?) see if the new EA tests pass for the new arrays. Tricky, since they aren't allowed internally. g
- [ ] failing dtype validation tests

This is going to fail. Going offline for a bit, but putting this up right now.

I'm going to clean up the changes in `internals` and the series constructor.