More tslibs TODOS: small optimizations, trim namespaces
A few methods of `Timedelta` and `Timestamp` that don't need to be user-facing are changed from `cpdef` to `cdef`.  To make the wheels turn, `Timestamp` methods that _call_ those methods are moved from `Timestamp` to `_Timestamp`.

Stronger typing for `_Timestamp. _get_start_end_field` and a couple others.

`@cython.final` supposedly allows for a small optimization because cython does not need to check at runtime whether a method has been overridden.  Applied to `Timestamp.__add__`, `Timestamp.__sub__`

