Auto merge of #32565 - tbu-:pr_cell_as_mut, r=alexcrichton
Auto merge of #32565 - tbu-:pr_cell_as_mut, r=alexcrichton

Add `as_mut` methods to the `std::cell` structs

This is safe since the borrow checking ensures that we have the only
mutable reference to the struct, thus we can safely borrow its interior.

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