WaitMap
## Summary This is an attempt to https://github.com/astral-sh/puffin/pull/1163 by removing the `WaitMap` and gaining more granular control over the values that we hold over `await` boundaries.
OnceMap
## Summary Ensures that if an operation is cancelled in one thread, we propagate it to others rather than panicking. Related to https://github.com/astral-sh/puffin/issues/1005.
## Summary This is extremely generic (like `WaitMap`), and I want to use it in the cache.