try_join_next for JoinSet

[jhelwig]

As pointed out in the discussion in #6157, JoinSet appears to be missing a method equivalent to try_recv on mpsc::Channel. There is poll_join_next, but this appears to be intended for when manually implementing the Future trait, as it requires access to a Context.

It is possible to maintain a set of JoinHandle, and periodically check is_finished on each, where there might be more JoinHandle added to the set that is being tracked between checks. However, it feels like JoinSet is also intended for this use case, and would be a much more ergonomic abstraction over having to manually do this bookkeeping.

[Darksonn]

We can add try_join_next.