Add modulo expressions, parsing, and conversion rules to Minion.

The undefinedness semantics for modulo are similar to those for
division: it requires a bubble rule, asserting that for x % y, y!=0, to
make it safe.

Due to their similarity, the test cases for modulo in this patch are
based on the division ones.

See also: conjure-cp#454 (implementation of division rules)

Closes: conjure-cp#456

Rename DivEq to DivEqUndefZero and ModEq to ModulusEqUndefZero. The
latter names match more closely to the underlying Minion constraints
being used, reflecting that we are using the undefzero variants of both