Rollup merge of #60404 - lo48576:borrow-mut-for-string, r=sfackler
Rollup merge of #60404 - lo48576:borrow-mut-for-string, r=sfackler

Implement `BorrowMut<str>` for `String`

Closes rust-lang/rfcs#1282.

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