Removes intertwining, since it is slow and showing up in profiles

Intertwining was a nice feature, but it slows down hyper significantly,
so it is being removed.

There is some fallout from this, mainly that Incoming has had its type
parameter changed to `<A = HttpAcceptor>` and Handler receiving one
bounded with `A: NetworkAcceptor`.

[breaking-change]

Fixes #112

benches/server.rs

@@ -31,7 +31,7 @@ fn bench_hyper(b: &mut Bencher) {
     let server = hyper::Server::http(Ipv4Addr(127, 0, 0, 1), 0);
     let mut listener = server.listen(hyper_handle).unwrap();
 
-    let url = hyper::Url::parse(format!("http://{}", listener.sockets[0]).as_slice()).unwrap();
+    let url = hyper::Url::parse(format!("http://{}", listener.socket).as_slice()).unwrap();
     b.iter(|| request(url.clone()));
     listener.close().unwrap();
 }