Pickle support can be cleaned up

[jamadden]

There's some gnarly code that works to ensure that C and Python versions of objects can be pickled and unpickled the same, especially so that unpickling a BTree always uses the currently available/selected implementation (C or Python).

I suspect that the reductions to boilerplate should make this much simpler.

BTrees/BTrees/_module_builder.py

Lines 49 to 52 in d49ff39

	# XXX: Consider defining these with their natural names
	# now and only aliasing them to 'Py' instead of the
	# opposite. That should make pickling easier.
	name = prefix + base_name + 'Py'