Add `stream.now()` method for allowing the user to produce `StreamInstant`s

[mitchmindtree]

As of #397 the callback now emits timestamps associated with:

1. The moment the callback is called and
2. The moment audio is delivered/received to/from the device.

As mentioned in #363, it would be very useful to allow the user to retrieve timestamps from the same source of time as used by the stream to produce these timestamps. That work did not make it into #397, so I've opened this as a reminder to address this step.

[FeldrinH]

Is there any news on this? This would be very useful for synchronizing audio playback with other things (video playback, stage lighting etc).