6195: Shorten iterators in associated params r=matklad a=SomeoneToIgnore Applies the same iterator-shortening logic to the iterator associated types, recursively. Before:  After: <img width="1192" alt="image" src="https://user-images.githubusercontent.com/2690773/95662894-e9038080-0b42-11eb-897d-527571ccac58.png"> Co-authored-by: Kirill Bulatov <mail4score@gmail.com>