auto merge of #20490 : japaric/rust/assoc-types, r=aturon

auto merge of #20490 : japaric/rust/assoc-types, r=aturon

closes #20486
closes #20474
closes #20441

[breaking-change]

The `Index[Mut]` traits now have one less input parameter, as the return type of the indexing operation is an associated type. This breaks all existing implementations.

---

binop traits (`Add`, `Sub`, etc) now have an associated type for their return type. Also, the RHS input parameter now defaults to `Self` (except for the `Shl` and `Shr` traits). For example, the `Add` trait now looks like this:

``` rust
trait Add<Rhs=Self> {
    type Output;

    fn add(self, Rhs) -> Self::Output;
}
```

The `Neg` and `Not` traits now also have an associated type for their return type.

This breaks all existing implementations of these traits.

---
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.