Trace sorting: Make q_arith boolean

[ledwards2225]

q_arith can currently take arbitrary values. We actively use q_arith = 0,1,2,3. (Comments in code suggest that at one time there was some use for q_arith > 3 but this doesn't seem to be used anywhere). This is an obstacle to representing selectors as boolean and/or as ranges rather than polynomials. I discussed this with Zac and he proposed the following to get rid of this:

• For q_arith = 3, simply do the less efficient thing and add an additional gate wherever this is used. (Currently only in nnf addition and subraction).
• For q_arith = 2, add a new selector q_5 that simply scales w_4_shift. This is as opposed to adding a whole new gate selector e.g. q_arith_2 which would require an entirely new "block", which has high cost once blocks are of a large fixed size.