Update stringed macro, add midi-poly-player compatible stringed instrument.

Macro updates:
- fix input names so single string instrument can work with midi-poly-player.
- add documentation string for the generated stringed instruments.
- add/use now + epsilon to eliminate most "late" messages.

Create single string instrument "ektara" & midi-poly-player example in comments.  Now, this works to allow you to play a midi keyboard "guitar":

    (def mpp (midi-poly-player (partial ektara :gate 1)))