Update ADSR exponent curves and fix some small issues in ADSR

These formulas seem to accurately reproduce the heights from a PSP.

There are a few other things to change still.  I tested bent a while ago and it seemed to be inaccurate as well, and keyon seems to take 32 samples to "kick in" (before which it mixes as zeroes.)  But I need to do more tests.

@gid15, fyi - the original code here came from JPCSP.  This should more accurately reproduce the curve.

Hopefully there are no regressions I missed.

-[Unknown]