`std::vec::IntoIter::as_mut_slice` borrows `&self`, returns `&mut` of contents.

[christophebiocca]

/u/dbaupp on reddit points out that this method can be called multiple times and result in getting multiple &mut [T] of the exact same data.

[brson]

Awesome!

[brson]

I suggest we change it to &mut and run it through crater to see the impact. We can change it or immediately deprecate it. We can make a point release or wait until 1.16.