std: Fix IntoIter::as_mut_slice's signature
std: Fix IntoIter::as_mut_slice's signature

This was intended to require `&mut self`, not `&self`, otherwise it's unsound!

Closes #39465

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