Separate RunNow and Setup

Hi,

After experimenting a bith with Amethyst, I started wondering why the `System` trait is responsible for both `setup` and `run` (+`dispose`). I programmed my setup to fetch a resource, then store it as a struct member, but I am forced to wrap it inside an `Option` and unwrap it upon each call of `run`.
Ideally I would have the `System` trait not having the `run` or `dispose` method, but move those to another trait. The `setup` would then consume the system and return an instance of this new trait.
In this way there will be no more need for me to wrap the resource during setup.

I tried to tinker with shred's code a bit myself, and it becomes clear that this is not a minor code change. Obviously it would break the API as well. So my question is : will this concept be in a future release ?