Add an async implementation of Queue

[datdenkikniet]

This PR adds an async version/wrapper around spsc::Queue with fallible interfaces that can be used in sync contexts.

Currently it uses a (non-public) single slot queue to store wakers.

We've been running this version of heapless for 2 months now and it's worked well so far.

[Dirbaio]

This should replace the previous waker instead of spinning the CPU at 100%.

Also, cloning wakers is moderately expensive in many executors (it does atomic refcount ops. Not in embassy or RTIC though), so it's nice to avoid cloning if possible. The way you do it is:

• If no waker registered, clone and register it.
• If a waker is registered:
  • If old_waker.will_wake(new_waker) do nothing. This saves a clone.
  • Otherwise, drop the old one, clone the new one and register it.

[korken89]

Hi, I think heapless should have an async module in where we can look at adding async specific data structures. Maybe @Dirbaio also has some from embassy that would make sense to put here?