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/tests/sync_watch.rs

@@ -195,7 +195,7 @@ fn poll_close() {
 
 #[test]
 fn stream_impl() {
-    use futures::StreamExt;
+    use tokio::stream::StreamExt;
 
     let (tx, mut rx) = watch::channel("one");