Add early-return for bdd_exists and bdd_forall

[SSoelvsten]

Consider the following piece of code in quant_rec (buddy/src/bddop.c)

   PUSHREF( quant_rec(LOW(r)) );
   PUSHREF( quant_rec(HIGH(r)) );
   
   if (INVARSET(LEVEL(r)))
      res = apply_rec(READREF(2), READREF(1));
   else
      res = bdd_makenode(LEVEL(r), READREF(2), READREF(1));

We can entirely circumvent the quant_rec(HIGH(r)) if

1. The quant_rec(LOW(r)) returned a terminal that shortcuts the given operator (see the base cases for apply_rec in the same folder for how to do so).
2. INVARSET(LEVEL(r)) is true