Add an mpmc concurrent queue

[RalfJung]

std currently has an MPSC queue, but the underlying implementation is MPMC, so it'd make sense to expose an MPMC queue as well. @rust-lang/libs-api has approved the general idea of exposing an MPMC queue.

The next step is to sketch the concrete API that would be added; some of the open questions are mentioned here. An ACP should then be filed for that API.

[obeis]

@rustbot claim