It can be very annoying if we want to use something like boolval, but have to define an entire anonymous function to pass it in. This PR adds support for tpassing it in directly as a string, like is posible with User::registerPreference.
Because invokable class objects are not directly called and instead it's the callback wrapper that calls these objects, it's currently not possible to receive arguments by reference on an invokable class.
To fix this we pass the arguments by reference by default when calling the object in the callback wrapper.
