CoreAudio sink and source

[gavv]

This task is identical to #231, but for adding CoreAudio native backend for macOS.

Three classes should be added to target_coreaudio:

• CoreaudioBackend
• CoreaudioSink
• CoreaudioSource

The implementation should use AudioUnit.framework from CoreAudio, a low-level audio API allowing to achieve the lowest possible latency.

[ghost]

Hi Victor, can I pick this up?

[gavv]

@nythoang Sure, you're welcome.