Allow assignments and update operators within assignments and update operators ++/--

[edemaine]

Sequel to #348 (further advancing #317) to also support update operators ++/--. Some fun consequences like ++x++ → ++x, x++. Also realized that x++ *= 2 should probably compile to x *= 2, x++, i.e., putting the inner assignment after instead of before the outer assignment, to preserve typical post-++ semantics.