dust2 rate density

Generates random impulses from -1 to +1.  The `density' is in
impulses per second.

> do { n <- dust2 AR 200
>    ; audition (out 0 (n * 0.5)) }

> let d = xLine KR 20000 2 10 RemoveSynth
> in audition . (out 0 ) . (* 0.15) =<< dust2 AR d