Add error when a constructor is used for a primitive argument.

Add testing for errors.

tests/LitConCase.newt.fail

@@ -0,0 +1,9 @@
+*** Process tests/LitConCase.newt
+module LitConCase
+ERROR at tests/LitConCase.newt:(7, 5): expected Prim.Int
+  foo : Int → Unit
+  foo 0 = MkUnit
+  foo MkUnit = MkUnit
+      ^
+
+Compile failed