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-test/src/task.rs

@@ -2,7 +2,6 @@
 
 #![allow(clippy::mutex_atomic)]
 
-use futures_core::Stream;
 use std::future::Future;
 use std::mem;
 use std::ops;
@@ -10,6 +9,8 @@ use std::pin::Pin;
 use std::sync::{Arc, Condvar, Mutex};
 use std::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};
 
+use tokio::stream::Stream;
+
 /// TOOD: dox
 pub fn spawn<T>(task: T) -> Spawn<T> {
     Spawn {