saugns v0.3.7. New channel mixing (panning) syntax.

The old P "keyword" for panning has been replaced.
Note that more changes remain to be done to remove
the old behavior of grouping the parameter setting
for all operators on one line. Place each on a new
line in the meantime to give it its own value.

Syntax changes:
 * Replace panning 'P' keyword with channel mixing parameter
   'c' and 'S c' for default. The scale changes from 0.0-1.0
   left-right to (-1.0) left, 0.0 center, 1.0 right. And the
   letters 'C', 'L', and 'R' become named constants for 0.0,
   (-1.0), and 1.0.

Some additional refactoring is done,
for some more lower-hanging fruit in
ideas from the 2020-06 "mgensys" redesign version.

The commit sequence is expanded a bit further down
for further work toward the redesign
goals, with notes on remaining bugs.
This tag has no release notes.