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.

examples/tinydb.rs

@@ -42,9 +42,10 @@
 #![warn(rust_2018_idioms)]
 
 use tokio::net::TcpListener;
+use tokio::stream::StreamExt;
 use tokio_util::codec::{Framed, LinesCodec};
 
-use futures::{SinkExt, StreamExt};
+use futures::SinkExt;
 use std::collections::HashMap;
 use std::env;
 use std::error::Error;