Replace `default_callback` function with `nullptr` and check for null…

Replace `default_callback` function with `nullptr` and check for null callback function before calling it. This is cleaner and better performing.

support enum

construct enum type to basic_json (proxy by int type)
```
    enum { t = 0 };
    json  j = json::array();
    j.push_back(t);

    j.push_back(json::object({
         {"game_type", t}
    }));
```

Fix performance regression introduced with the parsing callback feature.