(tokio-util JoinMap) Remove raw-entry feature in favour of HashTable API.

[conradludgate]

Motivation

While rebasing #7075, I noticed the change from #7219 which updates hashbrown, requiring the addition of the raw-entry feature. That inspired me to take a look at the actual JoinMap impl and I thought it could be improved by using HashTable instead.

Solution

Replacing the tasks_by_key map with a HashTable, we unlock a few benefits:

1. When searching by ID, we don't need to store the id in tasks_by_key, since we can fetch the ID from the AbortHandle.
2. We don't need to duplicate the S: BuildHasher, as we are in-control over the hash-function, we can fetch the existing hasher from hashes_by_task.

However, removing the ID from tasks_by_key will slow down those lookups (only in join_next()), as they now need a vtable lookup, rather than comparing against the id already in the current cache line.

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I don't feel strongly about this PR, I'm just quite fond of the HashTable API and thought I'd try it here.