audio playback with CoreAudio (HAL)

First working coreaudio hal implementation

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.

Somehow I get the sense that Apple would prefer me to use the AUHAL
audio unit.

TODO: better error handing (those printfs are ugly)
TODO: timing information
now that coreaudio is working, clean up the impl.