Added serialize/deserialize DSL keywords

These keywords would use the serializer engine, which is set on the app. That way, the Dispatcher can identify possible serialization/deserialization errors.

Current to_json, to_yaml, from_json etc methods all create a new object, which will then be used for the serialization job. But the error checking is done only on the $app->engine('serializer')

These keywords would also use the serializer, which is configured in the app.