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/src/codec/framed.rs

@@ -3,10 +3,9 @@ use crate::codec::encoder::Encoder;
 use crate::codec::framed_read::{framed_read2, framed_read2_with_buffer, FramedRead2};
 use crate::codec::framed_write::{framed_write2, framed_write2_with_buffer, FramedWrite2};
 
-use tokio::io::{AsyncBufRead, AsyncRead, AsyncWrite};
+use tokio::{io::{AsyncBufRead, AsyncRead, AsyncWrite}, stream::Stream};
 
 use bytes::BytesMut;
-use futures_core::Stream;
 use futures_sink::Sink;
 use pin_project_lite::pin_project;
 use std::fmt;

tokio-util/src/codec/framed_read.rs

@@ -1,10 +1,9 @@
 use crate::codec::framed::{Fuse, ProjectFuse};
 use crate::codec::Decoder;
 
-use tokio::io::AsyncRead;
+use tokio::{io::AsyncRead, stream::Stream};
 
 use bytes::BytesMut;
-use futures_core::Stream;
 use futures_sink::Sink;
 use log::trace;
 use pin_project_lite::pin_project;

tokio-util/src/codec/framed_write.rs

@@ -2,10 +2,10 @@ use crate::codec::decoder::Decoder;
 use crate::codec::encoder::Encoder;
 use crate::codec::framed::{Fuse, ProjectFuse};
 
-use tokio::io::{AsyncBufRead, AsyncRead, AsyncWrite};
+use tokio::{io::{AsyncBufRead, AsyncRead, AsyncWrite}, stream::Stream};
 
 use bytes::BytesMut;
-use futures_core::{ready, Stream};
+use futures_core::ready;
 use futures_sink::Sink;
 use log::trace;
 use pin_project_lite::pin_project;

tokio-util/src/codec/mod.rs

@@ -6,8 +6,8 @@
 //!
 //! [`AsyncRead`]: https://docs.rs/tokio/*/tokio/io/trait.AsyncRead.html
 //! [`AsyncWrite`]: https://docs.rs/tokio/*/tokio/io/trait.AsyncWrite.html
+//! [`Stream`]: https://docs.rs/tokio/*/tokio/stream/trait.Stream.html
 //! [`Sink`]: https://docs.rs/futures-sink/*/futures_sink/trait.Sink.html
-//! [`Stream`]: https://docs.rs/futures-core/*/futures_core/stream/trait.Stream.html
 
 mod bytes_codec;
 pub use self::bytes_codec::BytesCodec;