Fix type error in assert function

The string module is not defined when RuntimeProg is compiled. Therefore `String.^` could not be used.

basis/RuntimeProgScript.sml

@@ -47,7 +47,7 @@ val _ = process_topdecs `
   fun assert cond msg =
   if (cond)
   then ()
-  else (debugMsg (String.^ ("Assertion Failure: ") (msg));
+  else (debugMsg (msg);
         abort());`
   |> append_prog;