Add a str::from_utf8_prefix method

I want to take a slice of bytes and get the longest prefix of the slice which is valid utf8. My use case is that I'm reading utf8 data from the network but the read buffer might not be full, so it might contain a partial codepoint at the end. Also, if the remote host sends invalid utf8 it would be useful to be able to process any valid utf8 they send beforehand.

It's trivial to implement this function using Utf8Error::valid_up_to, but it requires unsafe code to avoid performing validation twice:

fn from_utf8_prefix(bytes: &[u8]) -> &str {
    match str::from_uf8(bytes) {
        Ok(s) => s,
        Err(err) => {
            unsafe {
                str::from_utf8_unchecked(&bytes[..err.valid_up_to()])
            }
        },
    }
}

This is even what the example code for Utf8Error does.

Since the standard library already contains the machinery for attempting utf8 validation and getting the index at which it fails, and since forcing people to write unsafe code for something trivial is generally a bad thing, I think this function should exist in std::str along-side from_utf8 and friends.

Start with the issue discussion and the linked Utf8Error::valid_up_to documentation, then review how this RFC repository handles proposals for std::str APIs. A complete contribution would need a resolved proposal for the from_utf8_prefix behavior and its interaction with invalid or incomplete UTF-8.