util: implement JoinMap

[hawkw]

Depends on #4630.

Motivation

In many cases, it is desirable to spawn a set of tasks associated with
keys, with the ability to cancel them by key. As an example use case for
this sort of thing, see Tower's ReadyCache type.

Now that PR #4530 adds a way of cancelling tasks in a
tokio::task::JoinSet, we can implement a map-like API based on the
same IdleNotifiedSet primitive.

Solution

This PR adds an implementation of a JoinMap type to
tokio_util::task, using the JoinSet type from tokio::task, the
AbortHandle type added in #4530, and the new task IDs added in #4630.

Individual tasks can be aborted by key using the JoinMap::abort
method, and a set of tasks whose key match a given predicate can be
aborted using JoinMap::abort_matching.

When tasks complete, JoinMap::join_one returns their associated key
alongside the output from the spawned future, or the key and the
JoinError if the task did not complete successfully.

Overall, I think the way this works is pretty straightforward; much of
this PR is just API boilerplate to implement the union of applicable
APIs from JoinSet and HashMap. Unlike previous iterations on the
JoinMap API (e.g. #4538), this version is implemented entirely in
tokio_util, using only public APIs from the tokio crate. Currently,
the required tokio APIs are unstable, but implementing JoinMap in
tokio-util means we will never have to make stability commitments for
the JoinMap API itself.