mantissaMask in bits

Masks off bits in the mantissa of the floating point sample
value. This introduces a quantization noise, but is less severe
than linearly quantizing the signal.

in - input signal
bits - the number of mantissa bits to preserve. a number from 0 to 23.

> let s = sinOsc AR (sinOsc KR 0.2 0 * 400 + 500) 0 * 0.4
> in audition (out 0 (mantissaMask s 3))