Fixed: Formula::Exists::Rectify() didn't register variables.

The bug implied that
  Ex x P(x) v Ex Q(x)
would be normalized to
  Ex x Ex x (P(x) v Q(x))
because Rectify(Ex x P(x)) did not register the variable x and
consequentially Rectify(Ex x Q(x)) didn't know it was registered
already.

Also removed the wrong assertion that in Formula::Bel::FreeVars() that
alpha and (~alpha v beta) must have the same free variables.