Reimplement the alsa_mixer module using the low level alsa_ctl implementation

[asabil]

libasound2 implements alsa_mixer using alsa_ctl, alsa_ctl being the actual low level interface used to communicate with the kernel driver. alsa_mixer provides an abstraction over the alsa controls, by combining for example the left and right channels of a low level control (which are separate) into a single control.

It might be interesting to consider re-implementing a replacement high level mixer interface instead of relying on libasound2's mixer implementation.