Update examples

This updates the examples to showcase the scope of anthem’s feature set.
New examples are added concerning placeholders, hiding predicates, and
simplifications related to integer variables.

examples/simple-external-show.lp

@@ -0,0 +1,9 @@
+s(X) :- p(X).
+s(X) :- q(X).
+u(X) :- r(X), not s(X).
+
+#show u/1.
+
+#external p(1).
+#external q(1).
+#external r(1).