expRand lo hi

Generates a single random float value in an exponential
distributions from `lo' to `hi'.

> import Sound.SC3.ID

> let { a = line KR 0.5 0 0.01 RemoveSynth
>     ; f = expRand 'a' 100.0 8000.0 }
> in audition (out 0 (fSinOsc AR f 0 * a))