slice: Stabilize IterMut::as_slice.
slice: Stabilize IterMut::as_slice.

Much like #72584.

As per #58957 there's no blocker for this, and I wanted to use this
today :-)

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust