It lets us re-use the same buffer for playing multiple files.
This also allows us to control the size of the buffer. Half of the
buffer will be used for the fist, and half for the second internal
buffer.
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`.
