breaks out SET ADD/SUB contraction code to optimization section

Adds a new snippet that contracts SET(x) ADD(1)/SUB(1) to
SET(inc(x)/dec(x)) and removes that same functionality from the ADD
and SUB bytecode compilation. The latter is just a revert to commit
da32544a7c2e0116fe4cc86717f3feb029f8dc74, that implemented SET(x) in
the frontend.