Emulate the APU (Audio)

[squidboylan]

The APU module should be a module created similarly to how the other modules represent emulated hardware, particularly the timer would probably be a useful reference as it is a simple module and it is easy to understand.

This may require some timing work to make sure that audio can be emulated with the correct timing (which is important for the obvious reason).

This should also have unit tests where possible.