Replace PyFunction.code PyMutex with PyAtomicRef for lock-free reads by youknowone · Pull Request #7317 · RustPython/RustPython

…ustPython#7317)

Change the code field from PyMutex<PyRef<PyCode>> to PyAtomicRef<PyCode>,
eliminating mutex lock/unlock on every function call. The setter uses
swap_to_temporary_refs for safe deferred drop of the old code object.