AudioFrame doesn't accept negative indexes or slices

[microbit-carlos]

Tested with v2.1.2 and the latest version (at the time of writing) of the recording & playback branch: 0b06914

We can use positive array indexes to access and modify the data inside an AudioFrame, but not a negative index to access.

>>> af = audio.AudioFrame()
>>> af[len(af) - 1] = 123
>>> af[-1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: index out of bounds
>>> af[1:]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't convert slice to int
>>> af[1:2]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't convert slice to int
>>> 

Technically we could use a memoryview for this, but it's not a well know feature, so even if we documented well and add it to the examples it might not be used as much.
We also have issue #187 released to memoryview.

If we wanted to enabled this for AudioFrame we should probably enable it for V1 as well. Is the performance/size penalty for this significant?

[microbit-carlos]

This will be covered as well with this new proposal:

• AudioFrame proposal: Reference external buffer  #205

[microbit-carlos]

No need to implement this in AudioFrame, just need to make sure it is in the AudioTrack implementation.

[dpgeorge]

AudioTrack's should now be able to be indexed with negative numbers.