pv_BinShift buffer stretch shift

Shift and scale the positions of the bins.  Can be used as a very
crude frequency shifter/scaler.  Shifts the leftmost bin at `buffer'
by `shift' places, the distance between subsequent bins is `stretch'.

> withSC3 (\fd -> async fd (b_alloc 10 2048 1))

> let { x  = mouseX KR (-10) 100 Linear 0.1
>     ; y  = mouseY KR 1 4 Linear 0.1
>     ; s0 = sinOsc KR 0.08 0 * 6 + 6.2
>     ; s1 = sinOsc KR (squared s0) 0 * 100 + 800
>     ; s2 = sinOsc AR s1 0
>     ; pv = pv_BinShift (fft' 10 s2) y x }
> in audition (out 0 (pan2 (ifft' pv) 0 0.1))