gh-59956: Allow the "Trashcan" Mechanism to Work Without a Thread State (gh-101209)

We've factored out a struct from the two PyThreadState fields. This accomplishes two things:

* make it clear that the trashcan-related code doesn't need any other parts of PyThreadState
* allows us to use the trashcan mechanism even when there isn't a "current" thread state

We still expect the caller to hold the GIL.

https://github.com/python/cpython/issues/59956