rust-cpython: drop self.borrow_mut() in favor of PySharedRef wrapper

rust-cpython: drop self.leak_immutable() in favor of PySharedRef wrapper

rust-cpython: add safe wrapper representing shared data borrowed from PyObject PySharedRef is a tempoary wrapper around PySharedRefCell. It provides safe functions for each shared data. $shared_accessor implements a safe method to construct PySharedRefCell. This allows us to add more than once PySharedRefCell to a Python object.

rust-cpython: move $leaked struct out of macro It wasn't easy to hack the $leaked struct since errors in macro would generate lots of compile errors. Let's make it a plain struct so we can easily extend it. PyLeakedRef keeps a more generic PyObject instead of the $name struct since it no longer has to call any specific methods implemented by the $name class. $leaked parameter in py_shared_iterator!() is kept for future change.