Async Ringbuffer

An asynchronous, fixed-capacity single-reader single-writer ring buffer that notifies the reader onces data becomes available, and notifies the writer once new space for data becomes available. This is done via the AsyncRead and AsyncWrite traits.

This crate is to AsyncRead and AsyncWrite what futures::unsync::spsc is to Stream and Sink (except that futures::unsync::spsc doesn't currently exist).