support for getting data off microphone

[mmoskal]

This adds:

• input.onSoundData(buf => ...)
• input.soundSamplingPeriod()
• input.setSoundSamplingPeriod()

The first one has to be implemented in pxt-microbit, not in extension, since it plugs into microphone pipeline before the level detector.

TODO:

• sim support

[jaustin]

@finneyj has a model for an input pipeline to make sure that these things don't cause breakages with the existing blocks/features and also doesn't break the oversampling switch as we go between mic only to mic and ADC. Worth discussing I think @mmoskal