Docstrings, de-duplicate EAMixin/DatetimeLikeIndex __new__ code
There's a lot of duplication in the constructors and constructor-helpers.  This starts to whittle that down, writes some docstrings long the way.

Also use `ExtensionOpsMixin` to define comparison operators on the EAMixin classes.

We determined that the DatetimeIndex._local_timestamps method had an unecessary monotonicy check, so took that out.