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/time_interval.rs

@@ -45,7 +45,7 @@ async fn usage() {
 
 #[tokio::test]
 async fn usage_stream() {
-    use futures::StreamExt;
+    use tokio::stream::StreamExt;
 
     let start = Instant::now();
     let mut interval = time::interval(ms(10));