Returning `&[u8]` support

[IFcoltransG]

For my use case I need byte slices composed mostly of ASCII with some arbitrary bytes thrown in as escapes. I hoped I could do something like this:

const data: &[u8] = formatcp!(b"Here is a 'string' with invalid UTF8 {INVALID_UNICODE} in it.", INVALID_UNICODE=[0xa0, 0xa1]);

To do this with const_format, I'd need a version of formatcp! that returns a byte slice literal, rather than a &str.

Feel free to close if creating &[u8] rather than &str is out of scope for these crates. As a workaround I'll use a slice concatenation crate, more like concat_bytes!(b"Here is a 'string' with invalid UTF8 ", INVALID_UNICODE, " in it.").