Just playing with the new *Reference types to see what patterns might arise.

This adds:

• PyObject.Reference internal property, that exposes the same pointer asobj, but in a way, that it is clear, that the reference is borrowed
• PyObject(BorrowedReference) constructor, that extracts the address from the reference andIncRefs it before storing in the instance
• a few of thePyList_* methods now explicitly takeBorrowedReference instead of raw pointers