faster delta_to_assoc/2

Example, previously:

   ?- time(solution(X > 1_000_000_000 /\ Y > 100*X)).
   % 100,874,446 inferences, 15.304 CPU in 17.675 seconds (87% CPU, 6591223 Lips)
   X = 1374389534,
   Y = 137438953464 .

Now:

   ?- time(solution(X > 1_000_000_000 /\ Y > 100*X)).
   % 49,808,959 inferences, 7.797 CPU in 9.143 seconds (85% CPU, 6387942 Lips)
   X = 1374389534,
   Y = 137438953408 .
allow existentially quantified variables to not occur in expressions

Example:

   ?- valid(exists(x,y=2)).
   false.