io: bring back split utility (#1521)

Bring back `split` utility as a free fn instead of a method on
`AsyncRead`. This utility wraps the `stream` in an `Arc` and uses mutual
exclusion to ensure correct access.

Additionally, the specialized `split_mut` fn on TcpStream and UdsStream
is promoted to `split`.

tokio-net/tests/uds_split.rs

@@ -11,10 +11,10 @@ use tokio::prelude::*;
 /// writing by reading to the end of stream on the other side of the connection.
 #[tokio::test]
 async fn split() -> std::io::Result<()> {
-    let (a, mut b) = UnixStream::pair()?;
+    let (mut a, mut b) = UnixStream::pair()?;
 
     let (mut a_read, mut a_write) = a.split();
-    let (mut b_read, mut b_write) = b.split_mut();
+    let (mut b_read, mut b_write) = b.split();
 
     let (a_response, b_response) = futures::future::try_join(
         send_recv_all(&mut a_read, &mut a_write, b"A"),