* First draft.
* Separate out mulmod constraint poly from the rest.
* Offset quot limbs when doing SUB* operations.
* Refactor constr_poly calculation.
* Clippy.
* Constrain the hi part of quot to be 0 or 1 for SUB* ops.
* Simplify storage and handling of negative quotient limbs.
* Remove unnecessary constant; move another to main file.
* Move constant back; cast properly.