Nit: `System::set_3d_rolloff_callback` is wrapped differently than all other callback APIs

Most callback APIs define a `trait ThisThingCallback` and take that as a turbofish, but `Rolloff3dCallback` is instead a type alias to `extern "system" fn` and passed as an optional function parameter.

Either `set_3d_rolloff_callback` should adapt to the style used everywhere else, or everywhere else should specify the callback as a function argument. Switching from `enum CallbackImpl {}` to `struct CallbackImpl;` makes this reasonable, but I still have a slight preference for the version which is clearly just type-level, since the callback functionality needs to be sent to FMOD as static function pointers (i.e. no `&self`).