tiRand lo hi trig

Generates a random integer value in uniform distribution from lo to
hi each time the trig signal changes from nonpositive to positive
values

> do { l <- tiRand (-1) 1 =<< dust KR 10
>    ; n <- pinkNoise AR
>    ; audition (out 0 (pan2 (n * 0.1) l 1)) }

> do { n <- tiRand 4 12 =<< dust KR 10
>    ; let f = n * 150 + (mce [0,1])
>      in audition (out 0 (sinOsc AR f 0 * 0.1)) }