Instead of keeping a *set* of untouchable variables in each
implication contraints, we keep a *range* of uniques for the
*touchable* variables of an implication.  This are precisely
the ones we would call the "existentials" if we were French.

It turns out that the code is more efficient, and vastly easier
to get right, than the set-based approach.

Fixes Trac #4355 among others