Rollup merge of #22313 - japaric:iter, r=aturon

Rollup merge of #22313 - japaric:iter, r=aturon

 `IntoIterator` now has an extra associated item:

``` rust
trait IntoIterator {
    type Item;
    type IntoIter: Iterator<Self=Self::Item>;
}
```

This lets you bind the iterator \"`Item`\" directly when writing generic functions:

``` rust
// hypothetical change, not included in this PR
impl Extend<T> for Vec<T> {
    // you can now write
    fn extend<I>(&mut self, it: I) where I: IntoIterator<Item=T> { .. }
    // instead of
    fn extend<I: IntoIterator>(&mut self, it: I) where I::IntoIter: Iterator<Item=T> { .. }
}
```

The downside is that now you have to write an extra associated type in your `IntoIterator` implementations:

``` diff
 impl<T> IntoIterator for Vec<T> {
+    type Item = T;
     type IntoIter = IntoIter<T>;

     fn into_iter(self) -> IntoIter<T> { .. }
 }
```

Because this breaks all downstream implementations of `IntoIterator`, this is a [breaking-change]

---

r? @aturon