The various datetime object __setstate__() methods are no longer public

The various datetime object __setstate__() methods are no longer public
(pickling no longer needs them, and immutable objects shouldn't have
visible __setstate__() methods regardless).  Rearranged the code to
put the internal setstate functions in the constructor sections.
Repaired the timedelta reduce() method, which was still producing
stuff that required a public timedelta.__setstate__() when unpickling.