stream: add next and map utility fn (#1962)

Introduces `StreamExt` trait. This trait will be used to add utility functions
to make working with streams easier. This patch includes two functions:

* `next`: a future returning the item in the stream.
* `map`: transform each item in the stream.

tokio-util/tests/framed.rs

@@ -1,11 +1,10 @@
 #![warn(rust_2018_idioms)]
 
-use tokio::prelude::*;
+use tokio::{prelude::*, stream::StreamExt};
 use tokio_test::assert_ok;
 use tokio_util::codec::{Decoder, Encoder, Framed, FramedParts};
 
 use bytes::{Buf, BufMut, BytesMut};
-use futures::StreamExt;
 use std::io::{self, Read};
 use std::pin::Pin;
 use std::task::{Context, Poll};

tokio-util/tests/udp.rs

@@ -1,4 +1,4 @@
-use tokio::net::UdpSocket;
+use tokio::{net::UdpSocket, stream::StreamExt};
 use tokio_util::codec::{Decoder, Encoder};
 use tokio_util::udp::UdpFramed;
 
@@ -6,7 +6,6 @@ use bytes::{BufMut, BytesMut};
 use futures::future::try_join;
 use futures::future::FutureExt;
 use futures::sink::SinkExt;
-use futures::stream::StreamExt;
 use std::io;
 
 #[tokio::test]