Stackless issue #139: separate pickling of generators and coroutines

Since upstream commit fc2f407 (bpo-32591) the memory layout of
generators and coroutines differs. Therefore it is no longer feasible to
share the pickling code.
This refactor the common parts of the reduce/setstate implementation
into support methods and support the new coroutine attribute cr_origin.

Author: Anselm Kruis