feat: to_{capitalized,titlecase}, to_{capitalized,titlecase}_into, make_ascii_{capitalized,titlecase}

[lopopolo]

I'm particularly interested in titlecase support since it is not provided by the std.

For the capitalize cases, I believe they could be implemented like:

trait ByteSlice {
    fn make_ascii_capitalized(&mut self) {
        if let Some((head, tail)) = self.as_bytes().split_first_mut() {
            head.make_ascii_uppercase();
            tail.make_ascii_lowercase();
        }
    }

    fn to_capitalized_into(&self, buf: &mut Vec<u8>) {
        // This allocation assumes that in the common case, capitalizing
        // and lowercasing `char`s do not change the length of the
        // `Vec`.
        buf.reserve(bytes.len());
        let mut bytes = self.as_bytes();
        match bstr::decode_utf8(bytes) {
            (Some(ch), size) => {
                // Converting a UTF-8 character to uppercase may yield
                // multiple codepoints.
                for ch in ch.to_uppercase() {
                    buf.push_char(ch)
                }
                bytes = &bytes[size..];
            }
            (None, size) if size == 0 => return,
            (None, size) => {
                let (substring, remainder) = bytes.split_at(size);
                buf.extend_from_slice(substring);
                bytes = remainder;
            }
        }
        while !bytes.is_empty() {
            let (ch, size) = bstr::decode_utf8(bytes);
            if let Some(ch) = ch {
                // Converting a UTF-8 character to lowercase may yield
                // multiple codepoints.
                for ch in ch.to_lowercase() {
                    buf.push_char(ch);
                }
                bytes = &bytes[size..];
            } else {
                let (substring, remainder) = bytes.split_at(size);
                buf.extend_from_slice(substring);
                bytes = remainder;
            }
        }
        self = replacement;
    }
}

The above code is MIT licensed and comes from https://github.com/artichoke/artichoke/blob/fc87277afda4ac3db59ea9c080bbb5f5170b6d10/spinoso-string/src/lib.rs#L1305-L1353.

[lopopolo]

I think my needs are a bit specialized here. I'm implementing what I need over here: https://github.com/artichoke/roe.

Sorry for the noise.