std::string throws typemaps

Go, Guile, Racket, Scilab: Add throws typemaps for std::string so that
thrown string exception messages can be seen.

Test all language for std::string throws typemaps

Examples/test-suite/ocaml/catches_strings_runme.ml

@@ -6,3 +6,9 @@ let _ =
     ignore (_StringsThrower_charstring (C_void)); assert false
   with Failure s ->
     assert (s = "charstring message")
+
+let _ =
+  try
+    ignore (_StringsThrower_stdstring (C_void)); assert false
+  with Failure s ->
+    assert (s = "stdstring message")