[Snek] [keith-packard/snek] f596ed: Share common formal assignment code in snek_func_push

[Keith Packard]

  Branch: refs/heads/master
  Home:   https://github.com/keith-packard/snek
  Commit: f596ed830f35fed313426bb341fab0b31aa99263
      https://github.com/keith-packard/snek/commit/f596ed830f35fed313426bb341fab0b31aa99263
  Author: Keith Packard <keithp at keithp.com>
  Date:   2019-12-01 (Sun, 01 Dec 2019)

  Changed paths:
    M snek-func.c

  Log Message:
  -----------
  Share common formal assignment code in snek_func_push

Most of the code for assigning formals is the same for named and
positional actuals. Create a single loop over the actuals and split
out the piece which figures out the name, either from the named
actual, or from the list of formals by position.

Also, add a pile of comments to this file.

Signed-off-by: Keith Packard <keithp at keithp.com>