[issue47215] Add "unstable" frame stack api

Change by Mark Shannon : -- keywords: +patch pull_requests: +30366 stage: needs patch -> patch review pull_request: https://github.com/python/cpython/pull/32303 ___ Python tracker

[issue47215] Add "unstable" frame stack api

New submission from Mark Shannon : We need to provide an API to create, swap and free frame stacks for greenlets. Since this is primarily for greenlets (and any other stackful coroutines libraries that want to use it) it will be "unstable". In this case, by "unstable" I mean: 1. Starts with