ring_buffer

A ring_buffer could be useful in situations where one wants a channel but back-pressure would be an issue.

It should have the same API as a channel.

On write, the writing coroutine should be yield to a reading coroutine if present.
If the ring-buffer is full the awaiting coroutine should post it-self to avoid starvation.

The ring_buffer could also have a `post` function, that posts a value and then the completion.
This function could also be thread-safe (since it posts internally) allowing for cross-thread notifications.

`T = void` would be a notifier, especially when of size 1.