REF: implement ABCTimestamp, ABCTimedelta, ABCTick
Motivation: to make more progress on getting DateOffset into cython, we need to be able to import Timestamp into liboffsets.  ATM that would induce circular dependencies.

This allows us to merge c_timestamp back into timestamps.pyx; the initial motivation for that was to be able to cimport _Timestamp into libconversion.

In a follow-up we can use this pattern to make a faster `is_interval` check (that doesnt have to go through python-space)