*WIP Async API for bbqueue

[lulf]

This is an initial draft of an async API as part of bbqueue with a test showing the working.

Status:

• Basic read and write API implementation + test
• Implement poll() that updates waker
• Implement notifying waker when data have been produced/consumed
• Tests of cancellation and waker logic
• API documentation

[MathiasKoch]

Is this PR completely stalled?

[lulf]

@MathiasKoch Yes, I didn't pursue this any further as I found I had no longer need for this. Feel free to take up the torch!