Implement sub primitive

compiler/src/l3/CPSValueRepresenter.scala

@@ -12,6 +12,10 @@ object CPSValueRepresenter extends (H.Tree => L.Tree) {
       tempLetP(CPS.Sub, Seq(Left(x), Right(1))) { x1 => 
         L.LetP(n, CPS.Add, Seq(x1, rewrite(y)), apply(body))
       }
+    case H.LetP(n, L3.IntSub, Seq(x, y), body) =>
+      tempLetP(CPS.Add, Seq(Left(x), Right(1))) { x1 =>
+        L.LetP(n, CPS.Sub, Seq(x1, rewrite(y)), apply(body))
+      }
 
   }