tokio-util: add AsyncRead/AsyncWrite passthrough for inspect

[cconstantine]

This commit allows you to use InspectReader and InspectWriter on types that implement both AsyncRead + AsyncWriter without losing those trait implementations.

Motivation

While implementing clients for tcp interfaces it can be helpful to inspect the bytes sent / received through a tokio::io::TcpStream. The existing InspectReader / InspectWriter structs are a good start, but using them causes the stream to lose its AsyncWrite / AsyncRead trait impls.

Solution

By adding implementations for AsyncRead to InspectWriter and AsyncWrite to InspectReader it is now possible to inspect bytes sent through a TcpStream (or any other T: AsyncRead+AsyncWrite) and still pass that stream onto another subsystem that expects a T: AsyncRead+AsyncWrite.

For example:

use tokio::io::TcpStream;
use tokio_util::io::{InspectReader, InspectWriter};

#[tokio::main]
async fn main() {
    let stream = TcpStream::connect("127.0.0.1:8080").await.unwrap();
    let stream = InspectReader::new(stream, |buf: &[u8]| ... );
    let stream = InspectWriter::new(stream, |buf: &[u8]| ...);
        
    SomeClient::new(stream).run();
}

[cconstantine]

The failing ci check appears to be unrelated to my changes.

[Darksonn]

You have some unnecessary trait bounds.

[Darksonn]

Thank you.