audio playback with CoreAudio (HAL)

Coreaudio HAL Implementation. Somehow I get the sense that Apple perfers
people to use the AUHAL audio unit.

We query for the prefered stereo channels, then configure the streams
containing those channels, and register the channels to write to for our
IO Proc.

The IO proc then finds its audiobuffers, query the client for samples,
then puts them into the hw buffer.

This implementation needs sometimes to show error information which
might be useful for the user to troubleshoot.

TODO: implement some error reporting in the runtime itself, so an user
can see them.
having fun with drones