* Remove the world module, as the base hook method has become much
  more powerful.
* Rename SceneHook to Hook, now Hook has a unique method to implement.
* You don't have to add any system yourself, now you have to add the
  HookPlugin plugin to your app.
* Move the API exclusively to a new SystemParam: HookingSceneSpawner.
  Please use that parameter to add and remove scenes that contain hooks.
* Moved the when_spawned run criteria to the is_scene_hooked
  function exposed at the root of the crate, the HookedSceneState
  system parameter or the SceneLoaded component. Please use any of
  those three instead of when_spawned.
* Now supports passing closures as hooks, instead of having to define
  a trait each time.
* Now supports adding multiple of the same scene! Doesn't handle
  hot-reloading, but that's alright since bevy's scene hot-reloading
  is currently broken anyway :D