feat(client): change connectors to return an impl Connection

Instead of returning a tuple `(impl AsyncRead + AsyncWrite, Connected)`,
this adds a new trait, `hyper::client::connect::Connection`, which
allows querying the connection type for a `Connected`.

BREAKING CHANGE: Connectors no longer return a tuple of
  `(T, Connected)`, but a single `T: Connection`.

examples/client.rs

@@ -3,8 +3,7 @@
 use std::env;
 use std::io::{self, Write};
 
-use hyper::Client;
-use futures_util::StreamExt;
+use hyper::{Client, body::HttpBody as _};
 
 // A simple type alias so as to DRY.
 type Result<T> = std::result::Result<T, Box<dyn std::error::Error + Send + Sync>>;