DTA/TDI init methods
(placeholder for now. will fill in later.)

These methods are starting to use sequence_to_td/dt64ns, which makes them parse strings, which some of use don't want. Those methods also do redundant type checking, which induces overhead over our ideal of a simple init.

xref https://github.com/pandas-dev/pandas/pull/24666 and the DTA issue I'll add later.

Not a blocker for the RC.