missing `AsMut<str>` implementation for `String`

[tspiteri]

String has an inherent method as_mut_str, and implements both DerefMut and BorrowMut<str>. Is there a reason why it does not implement AsMut<str>, or was it just an oversight?

[ExpHP]

It just goes to show how useless &mut str is that nobody was bothered by this until today. 😛

[estebank]

Tangentially related: #44695