further slim down Uniter

* dropped the .service field, which is used in a single call, at most once per run
* moved relationers and relationHooks behind new Relations interface

This is a straight refactoring and as such does not involve any new tests; I don't feel great about that, but I think  it's better to land it now and leave it exactly as badly tested as before than to spend more days putting everything in a perfect state.

(Review request: http://reviews.vapour.ws/r/606/)