Rollup merge of #68742 - tspiteri:string-as-mut, r=sfackler
Rollup merge of #68742 - tspiteri:string-as-mut, r=sfackler

implement AsMut<str> for String

Closes #68741.

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