Idea: pre-optimize `distinct` by pruning solved vars

[pvdz]

A distinct is perhaps the most expensive constraint that we have because it builds a "pyramid" of propagators. One distrinct on n variables causes n^2-1 propagators because it has to create an neq for every pair of variables (but not to itself). Being able to eliminate some means a potential reduction of a lot.

Now solved variables in a distinct are easily resolved. Say we have a var x which is solved, we search for the value of x in all the other vars. If we encounter it, we remove it from the domain. Then we drop this var from the list of vars for distinct. Repeat for all vars.

In the real world this may not help much, but there's at least one case in the curator right now that does something "silly" like distinct([1,1], [2,2], [3,3], [4,4], [5,5], [6,6], [7,7], [8,8], [9,9], [10,10], [11,11], [12,12], [13,13], [14,14], [15,15], [16,16], [17,17], [18,18]). That is a distinct on 18 vars, causing 18^2-1=323 propagators for "obviously" no good reason. I mean, there's probably a reason, but finitedomain should definitely eliminate these cases as soon as possible and the generator should reconsider whether it actually needs to do this in the first place.