Stabilise FromUtf8Error::as_bytes
Stabilise FromUtf8Error::as_bytes

Closes #40895.

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