[macOS] CoreAudio backend

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Add a native CoreAudio backend to replace portaudio on MacOS.

Some experiments have started in the coreaudio-backend branch.

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The experiments started promising, but failed.

Managed to record properly from some devices. But not from others. One of the issues seems to be in the way CoreAudio handles sample rates. Different devices offer different sample rates. And auto-conversion is not activated. Maybe it requires using the library differently. I tried reading as much documentation as possible, but they mostly disagree.

For now, I'll put this on hold, until I find better documentation or more motivation.