a playbuf alternative that loops and fade

I'm writing an implementation of XPlayBuf, that can loop, cross-fade on
trigger and fade on loop boundaries.
It mostly follows the spec at
https://github.com/supercollider/supercollider/wiki/PlayBuf-2017, with a
few important features missing for now (no UnitTest yet, no .kr yet, only
two fade shapes for now, no loops across buffer extremes yet, no resetPos).

If you want to join, comment, or just give it a try, it's at
https://github.com/elgiano/xplaybuf