Use assoc types in the *Iterator traits

[japaric]

Affected traits:

• Iterator { type Item }
• IteratorExt no input/output types, uses <Self as Iterator>::Item in its methods
• DoubleEndedIterator no input/output types, uses <Self as Iterator>::Item in its methods
• DoubleEndedIteratorExt no input/output types, uses <Self as Iterator>::Item in its methods
• RandomAccessIterator no input/output types
• ExactSizeIterator no input/output types, uses <Self as Iterator>::Item in its methods

This breaks all the implementations of these traits.

[breaking-change]

---

I was planning to use assoc types in the AdditiveIterator trait but that can't be done until this bug (#20400) is fixed.

r? @aturon

[aturon]

Other than a couple of possible mergers, this LGTM!

[bluss]

Is <Self as Iterator>::Item needed, doesn't Self::Item work?

[japaric]

@bluss Self::Item works if the assoc type is defined in the same trait, but it doesn't work with supertraits:

#![crate_type = "lib"]
#![feature(associated_types)]

trait Iterator {
    type Item;

    fn ok() -> Self::Item;
}

trait DoubleEndedIterator: Iterator {
    fn doesnt_work() -> Self::Item;
    fn works() -> <Self as Iterator>::Item;
}

Unless @nikomatsakis fixed it while I was sleeping ;-).

[nikomatsakis]

💛