Make PyObject serialization more flexible and performant

[saulshanabrook]

#366 changes how py-objects are stored to pickle them first, instead of storing them as references. We could follow up with a few more items to extend the support:

• Don't copy byte string in rust, just hold a view
• Allow setting custom pickler subclass. This would also let you re-implement something like we had before where objects are stored by reference
• Support buffers when pickling for zero copy ndarrays