Reimplement integer variable detection

This is a reimplementation of the integer variable detection procedure.
The idea is to iteratively assume variables to be noninteger, and to
prove that this would lead to a false or erroneous result. If the proof
is successful, the variable is integer as a consequence.

examples/schur-numbers.lp

@@ -1,7 +1,9 @@
+#show in/2.
+#external integer(n(0)).
+#external integer(r(0)).
+
 {in(1..n, 1..r)}.
 covered(I) :- in(I, S).
 
 :- I = 1..n, not covered(I).
 :- in(I, S), in(J, S), in(I + J, S).
-
-#show in/2.