Add standalone module internals.weak with data structures for caching

[eb8680]

Factored out of #743 to ease review.

This PR contains only the new internal-facing module effectful.internals.weak and its tests tests/test_internals_weak.py from #743. This module and its tests are self-contained and do not depend on any other part of effectful; no changes here affect the rest of the library in any way.

Most of the implementation of internals.weak.WeakIdKeyDictionary is copied from torch.utils.weak, although I've made some fixes and cleaned up types and docstrings. The things on top of that are new.

[jfeser]

I've pushed a failing test that should pass. I haven't read this code in detail, just put it through GPT.

[eb8680]

I fixed the test you pushed.

I haven't read this code in detail, just put it through GPT.

The nastiest part of the code is the implementation of WeakIdKeyDictionary (including _iterationGuard and WeakIdRef) which is a forked and lightly modified version of torch.utils.weak.WeakIdKeyDictionary and thus shouldn't need careful review. The other stuff in weak.py is more straightforward and largely sits on top of that. The tests were generated by Claude from a specification I wrote and reviewed with/for it, along with the tests from PyTorch for torch.utils.weak (which are not exactly comprehensive). I am never thrilled with Claude's test code but I think it covered everything in the spec.