whisper : add option to speed up the audio tempo by x2

whisper : add option to speed up the audio tempo by x2

Using a Phase Vocoder for speeding up the audio tempo by scaling down
the frequencies in the frequency domain.

This reduces the computation in the Encoder by a factor of 2.
The transcription accuracy is degraded, but for slow to normal speech -
it seems to be still very good.

I think this can find application for real-time transcription - i.e. the
"stream" example.