Add missing Iterator bound

[nikomatsakis]

The fixes from rust-lang/rust#27641 uncovered the fact that you are missing an Iterator bound here; otherwise, <T as Iterator>::Item doesn't make sense, since T: Iterator may not hold. In fact, now that a T: Iterator bound is given, you can just write T::Item instead.

[Emerentius]

The real bug here is that the trait is generic for no reason. What I really meant to write was Self::Item. Then it suffices that the trait extends Iterator.

Thanks for the notice!

[Emerentius]

Removed the offending generic bounds in 1e76a36.