CLN: datetimelike arrays: isort, small reorg
Big diff, few real changes.

Implement `__repr__` for DTA/TDA.  Shared with PeriodArray implementation.  Added truncation so we don't print all N elements.  Adds tests for all three subclasses.  Creates dedicated test files for test_datetimes and test_timedeltas

Some PeriodArray methods got shuffled into weird places, so this puts them back.
Collects wrapping-properties in DatetimeIndex.
isort on indexes/datetimelike, datetimes, timedeltas