Improve (Not) Equals propagation

[EmirDe]

Currently we only propagate if there is only one unassigned variable, but this could be improved for the case with two unassigned variables. In that case, we can remove values from one variable that have no corresponding value in the other variable. For example, if the constraint boils down to x - y = 0 (x = y), then we can intersect the two domains. Similarly if the constraint is x + y = 5 with x, y \in {1, 3, 4, 5}, then the values 3 and 5 can be propagated out.

todo check https://link.springer.com/chapter/10.1007/3-540-45349-0_34