[OCaml] Fix the wrapmacro test

Add a typecheck typemap for size_t and const size_t &.

Add the const qualifier to the typemaps for primitive reference
types.

Add multiple runtime tests.

Examples/test-suite/ocaml/global_vars_runme.ml

@@ -0,0 +1,15 @@
+open Swig
+open Global_vars
+
+_init '()
+
+let _ =
+  assert (_b '() as string = "string b");
+  assert (_b '("a string value") as string = "a string value");
+  assert (_b '() as string = "a string value");
+  assert (_x '() as int = 1234);
+  assert (_x '(9876) as int = 9876);
+  assert (_x '() as int = 9876);
+  assert (_Hi '() as int = 0);
+  assert (_Hola '() as int = 1);
+;;