Add back BorrowedBuf::filled_mut This is useful if you want to do some processing on the bytes while still using the BorrowedBuf. The API was removed in https://github.com/rust-lang/rust/pull/97015 with no explanation. The RFC also has it as part of its API, so this just seems like a mistake: [RFC](https://rust-lang.github.io/rfcs/2930-read-buf.html#:~:text=inline%5D%0A%20%20%20%20pub%20fn-,filled_mut,-(%26mut%20self)) ACP: https://github.com/rust-lang/libs-team/issues/139