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.

https://github.com/zopefoundation/BTrees/blob/d49ff39eb4b884eecd324ca0e106f46365fa12bb/BTrees/_module_builder.py#L49-L52