rust-cpython: remove useless PyRefMut wrapper

rust-cpython: drop manual management of mutably_borrowed RefCell::borrow() should guarantee there's no mutable borrow. As a follow up, maybe PySharedState can be a pure data structure + function. Most ref-sharing business has already been moved to PySharedRef* and PyLeaked*.

rust-cpython: leverage RefCell::borrow() to guarantee there's no mutable ref Since the underlying value can't be mutably borrowed by PyLeaked, we don't have to manage yet another mutably-borrowed state. We can just rely on the RefCell implementation. Maybe we can add try_leak_immutable(), but this patch doesn't in order to keep the patch series not too long.

rust-cpython: remove useless Option<$leaked> from py_shared_iterator We no longer need to carefully drop the iterator when it's consumed. Mutation is allowed even if the iterator exists. There's a minor behavior change: next(iter) may return/raise something other than StopIteration if it's called after the iterator has been fully consumed, and if the Rust object isn't a FusedIterator.

rust-cpython: allow mutation unless leaked reference is borrowed In other words, mutation is allowed while a Python iterator holding PyLeaked exists. The iterator will be invalidated instead. We still need a borrow_count to prevent mutation while leaked data is dereferenced in Rust world, but most leak_count business is superseded by the generation counter. decrease_leak_count(py, true) will be removed soon.

rust-cpython: add generation counter to leaked reference This counter increments on borrow_mut() to invalidate existing leaked references. This is modeled after the iterator invalidation in Python. The other checks will be adjusted by the subsequent patches.