[OCaml] Don't use argout typemaps by default for some reference types

The OCaml module's typemaps.i supplied argout typemaps for some
reference types (e.g. int &) by default, which was unintuitive and
inconsistent when compared with other modules. The argout_ref example
depended on this, so add a typemap to argout_ref/example.i.

Add multiple runtime tests that deal with references.

Examples/ocaml/argout_ref/example.i

@@ -1,6 +1,11 @@
 /* File : example.i */
 %module example
 
+%typemap(argout) (int &x, int &y) {
+  swig_result = caml_list_append(swig_result, caml_val_int(*$1));
+  swig_result = caml_list_append(swig_result, caml_val_int(*$2));
+}
+
 %{
 extern "C" void   factor(int &x, int &y);
 %}