Upstream `RefMutGuard` / `RefMutContainer` to PyO3

[davidhewitt]

Hello, I'm interested in helping tokenizers ship free-threaded Python wheels (#1784). I plan to open one or more PRs in the near future to help move that forward.

I was studying the RefMutGuard pattern and think it's a very interesting solution for safely allowing access to &mut T within a Python callback. I would be interested in generalizing this to have a similar pattern for &T and releasing this upstream in PyO3.

I wanted to seek your permission here first - while I expect the code will not be a verbatim copy of your implementation, it would be nice to credit original design to you in the PyO3 source and also I can help update tokenizers to use PyO3's implementation once we've shipped it.

Please let me know what you think!

[ArthurZucker]

Absolutely! This is fully open source 🫡

[davidhewitt]

👍 great! First PR up at #1901 and I've opened the upstream PyO3 issue in PyO3/pyo3#5664