Expose `http::byte_str::ByteStr` to the public API

[lexoooooo]

When using the HTTP1.1 protocol, we can convert the entire buffer into bytes:: Bytes, and then convert a portion of it into ByteStr for use (for example, as HeaderName) for zero-copy network programming.I think it's very useful and important

[lexoooooo]

We just need to restore the implementation TryFrom<Bytes>

[seanmonstar]

I don't think we need to expose the actual ByteStr type. But if there's parts that cannot be created via zero copy, we can add ways.

[fakeshadow]

I have a similar use case for ByteStr. For given PathAndQuery type it's possible to match on the path and slice the Bytes to avoid copy.

for example:

fn main() {
    use bytes::Bytes;
    use http::uri::PathAndQuery;
    use std::fmt;

    #[derive(Debug)]
    struct User {
        name: Bytes,
        age: Bytes,
    }

    impl fmt::Display for User {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            let name = std::str::from_utf8(self.name.as_ref()).unwrap();
            let age = std::str::from_utf8(self.age.as_ref()).unwrap();
            write!(f, "User {{ name: {name}, age: {age} }}")
        }
    }

    let path = PathAndQuery::from_static("/john/21"); 
    let bytes = Bytes::copy_from_slice(path.path().as_bytes()); // if path return ByteStr or Bytes this copy can be elimnated.
    let name = bytes.slice(1..=4);
    let age = bytes.slice(6..=7);

    println!("{}", User { name, age });
}