Use generic associated type in StdinMappers

[azriel91]

In GitLab by @azriel91 on Sep 26, 2018, 13:20

Generic associated types will allow you to specify associated types that contain lifetimes (similar thread):

pub trait StdinMapper {
    type Resources: for<'res> SystemData<'res>;
}

// other crate:
pub struct CharacterSelectionEventStdinMapper {
    type Resources<'res> = Read<'res, CharacterAssets>;
}

Currently GATs hasn't been stabilized, so we aren't able to parameterize the Resources associated type with the lifetime.
Pending rust-lang/rust #44265.