Fix two typos
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]
auto merge of #17213 : mo/rust/fix_typos, r=aturon
Fallout from renaming
auto merge of #17268 : aturon/rust/mut-conventions, r=alexcrichton

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. Additional details and motivation in the RFC.

Note that the iterator *type* names are not changed by this RFC; those are awaiting a separate RFC for standardization.

Closes #13660
Closes #16810

[breaking-change]
expose expect intrinsics + mark push slow path

This marks the slow path in the `Vec<T>` `push` function, improviding
code generation by moving it out of fast paths. The `push` function is
inlined in countless locations and not all of them order these blocks
appropriately without a hint. This improves the performance of the
microbenchmark in #15177 by 15% when dirty page purging is disabled.

Exposing the i1 version of the intrinsic for `bool` would be useful, but
the quirks of Rust's code generation prevent it from becoming branch
metadata so it has been left out for now.

A more sophisticated approach could be taken in the future (#11092) but
this provides a useful optimization feature today.