rust-cpython: make inner functions and structs of ref_sharing private Most of these methods were public because they had to be accessible from macro-generated functions. Some "unsafe" can be removed since we can guarantee the data consistency across non-public operations.

rust-cpython: keep Python<'a> token in PyRefMut This just clarifies that the GIL is obtained while PyRefMut is dereferenced, so there's no need of extra acquire_gil() to drop the reference.