[OCaml] Fix some of the OCaml examples

Update some of the OCaml examples so that they run successfully with
`make check-ocaml-examples` (important for the CI). Some of the
examples were written to depend on passed arguments.

Temporarily disable the broken std_string example.

Examples/ocaml/strings_test/example.i

@@ -11,4 +11,5 @@ using std::string;
 #include "example.h"
 %}
 
+%include "std_string.i"
 %include example.h