This change replaces the undocumented `ThingMixin`, and the unfortunately-named, poorly-considered idiom of using `installOn` with its subclasses, with a new, thoroughly documented and better-named mixin, `imaginary.enhancement.Enhancement`.
Furthermore, this updates `createCreator` to use `Enhancement`s rather than arbitrary powerups.
The tests and included systems now also provide a good example of how Imaginary application / game code should idiomatically fit into the framework.