impl ExactSize for vec::MoveItems
auto merge of #17266 : Gankro/rust/vec-move, r=alexcrichton

Seems to correctly report exact size, so it should claim to do so formally.
Align with _mut conventions

As per [RFC
52](https://github.com/rust-lang/rfcs/blob/master/active/0052-ownership-variants.md),
use `_mut` suffixes to mark mutable variants, and `into_iter` for moving
iterators.

[breaking-change]
Fallout from renaming