Add option to speed up the audio tempo by x2

[ggerganov]

ref #137

Using a Phase Vocoder for speeding up the audio tempo by scaling down the frequencies in the frequency domain.
Use the -su or --speed-up command line argument to enable it:

./main -m ./models/ggml-small.en.bin -f samples/gb0.wav -su

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.

Similar result is achieved with ffmpeg's atempo filter:

# speed-up tempo by factor of 2
ffmpeg -i samples/gb0.wav -filter_complex "[0:a]atempo=2.0[a]" -map "[a]" samples/gb0-fast.wav

./main -m ./models/ggml-small.en.bin -f samples/gb0-fast.wav