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.

whisper.h

@@ -202,6 +202,9 @@ extern "C" {
         float thold_ptsum;      // timestamp token sum probability threshold (~0.01)
         int   max_len;          // max segment length in characters
 
+        // [EXPERIMENTAL] speed-up techniques
+        bool speed_up; // speed-up the audio by 2x using Phase Vocoder
+
         const char * language;
 
         struct {