Skip unused variables.

[make-github-pseudonymous-again]

See discussion in #36. One way to solve this problem is to introduce a variable mapping for sat.from.parities and sat.from.signs.

[make-github-pseudonymous-again]

Could use this mapping to introduce a random permutation of the variables to be oblivious to a particular kind of adversary. Maybe this should be optional. Note that we might want to maintain the correct value for cardinality.count(sat.solve(...)). Or maybe we would have instance.assignments( certificate ) iterate over all corresponding assignments, certificate could have a 'any' flag -1. Then some sugary cardinality.count(sat.assignments(sat.solve(...))) would give the correct count. Or there should be a sat.count(sat.solve(...)). IDK