Precise timing based on AsyncProcessor (WIP)

[y-usuzumi]

Summary

This PR attempts to provide a unified adapter for CoreAudio and JACK.

• CoreAudio follows a synchronous workflow where you take care of calling MidiPort.write() at the correct time or give it a precise timestamp for playback later (in the unit of monotonic ticks based on mach_absolute_time.
• JACK gives you more flexible control by calling your registered callback at every process cycle (based on sample rate, buffer size and period). It is much easier to calculate the offset in the buffer to write base on the beat time.

When the "coreaudio" feature is enabled, a thread will be spawned that calls an audio-framework-agnostic AsyncProcessor at a fixed interval (based on sample rate and buffer size), in which a MIDIAdapter is passed which can be used to write MIDI notes.

When the "jack" feature is enabled, a callback will be registered to JACK that simply calls AsyncProcessor.on_process_cycle().

The MIDIAdapter has different implementations for CoreAudio and JACK (of course).