ability to open a duplex stream

The concept of opening a stream simultaneously for input and output exists, but libsoundio does not support it. It's one thing that PortAudio currently does better than libsoundio.

This likely entails an API overhaul and 2.0.0.

Once this is done and works correctly, the microphone example no longer would need a ring buffer, so it would be that much more efficient. And then the only usage of a ring buffer would be the dummy backend, and that can be easily replaced by something trivial (and doesn't waste time actually copying memory).

So this probably means deleting the ring buffer from the API and deleting the internal implementation. This is in service of libsoundio's goal of being a lightweight abstraction.