(or deinitialized, for those of us on this side of the pond)
Otherwise, a sequence like
```
audio = audiobusio.I2SOut(bit_clock=board.D6, word_select=board.D9, data=board.D10)
sine_wave_sample = audiocore.RawSample(sine_wave)
audio.play(sine_wave_sample, loop=True)
del audio
```
could free the memory associated with audio without stopping the
related background task. Later, when fresh objects are allocated within
a now-freed memory region, they can get overwritten in the background
task, leading to a hard crash.
This presumably can affect multiple I2S implementations, but it was
reported against the nRF one.
This evolves the API from 2.x (and breaks it). Playback devices are now
separate from the samples themselves. This allows for greater playback
flexibility. Two sample sources are audioio.RawSample and audioio.WaveFile.
They can both be mono or stereo. They can be output to audioio.AudioOut or
audiobusio.I2SOut.
Internally, the dma tracking has changed from a TC counting block transfers
to an interrupt generated by the block event sent to the EVSYS. This reduces
the overhead of each DMA transfer so multiple can occure without using up TCs.
Fixes#652. Fixes#522. Huge progress on #263
