[OCaml] Fix possible GC issues in generated director code

Make `classDirectorMethod()` generate `CAMLreturn_type()` or
`CAMLreturn0` when there are local variables of type `value`.

Examples/test-suite/ocaml/director_unroll_runme.ml

@@ -0,0 +1,15 @@
+open Swig
+open Director_unroll
+
+let director_unroll_MyFoo ob meth args =
+  match meth with
+    | "ping" -> C_string "MyFoo::ping()"
+    | _ -> (invoke ob) meth args
+
+let a = new_derived_object
+  new_Foo (director_unroll_MyFoo) '()
+
+let b = new_Bar '()
+let _ = b -> set (a)
+let c = b -> get ()
+let _ = assert ((a -> "&" () as int) = (c -> "&" () as int))