Testing performed: That the shipped .mpy files on a PyPortal (CP 4.x)
still work (play audio) with this branch, instead of erroring because
`WaveFile` can't be found in `audioio`.
Flash usage grew by 28 bytes. (I expected 24, there must be some other
effect on size/alignment that I didn't predict)
When nrf pwm audio is introduced, it will be called `audiopwmio`. To
enable code sharing with the existing (dac-based) `audioio`, factor
the sample and mixer types to `audiocore`.
INCOMPATIBLE CHANGE: Now, `Mixer`, `RawSample` and `WaveFile` must
be imported from `audiocore`, not `audioio`.