Declare enums explicitly, fix type hints (#74)

* Declare enums explicitly, fix hints

* Align spec with existing implementations

* Format the spec with black

* Remove NaT (not-a-datetime) from the `USE_NAN` description.

This address the review comment that NaT is not a thing outside of
NumPy. Hence for not-a-datetime, all implementers should be using
sentinel values, because those are explicit.

* Change `Column.size` from a property to a method, and remove the option to return `None`

This to indicate that using `.size()` may now trigger a computation
(properties should be O(1) look-ups).

Co-authored-by: Vasily Litvinov <[email protected]>
Co-authored-by: Ralf Gommers <[email protected]>

Author: vnlitvinov