[REF] Move comparison methods to EAMixins, share code
Changes an old usage numpy's C API that is deprecated.  This won't get rid of the warnings because cython hasn't changed it, but still.

Also stops making copies of offsets since they are now immutable.

Handles a handful of changes requested in the last pass: de-privatizes _quarter_to_myear (plus bonus docstring), renames _generate --> _generate_range, comments in is_list_like

renames arrays.timedelta --> arrays.timedeltas to match core.indexes

Implements comparison methods in DatetimeArray and TimedeltaArray, cleans up some Index code that is no longer needed as a result.

Makes some progress on sharing code between TDI and DTI constructors (most of which we want to move up to the array classes)