io: add missing utility functions (#1632)

The standard library's `io` module has small utilities such as `repeat`,
`empty`, and `sink`, which return `Read` and `Write` implementations.
These can come in handy in some circiumstances. `tokio::io` has no
equivalents that implement `AsyncRead`/`AsyncWrite`.

This commit adds `repeat`, `empty`, and `sink` helpers to `tokio::io`.

tokio/src/io.rs

@@ -41,8 +41,8 @@
 pub use tokio_fs::{stderr, stdin, stdout, Stderr, Stdin, Stdout};
 pub use tokio_io::split::split;
 pub use tokio_io::{
-    AsyncBufRead, AsyncBufReadExt, AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt, BufReader,
-    BufWriter,
+    empty, repeat, sink, AsyncBufRead, AsyncBufReadExt, AsyncRead, AsyncReadExt, AsyncWrite,
+    AsyncWriteExt, BufReader, BufWriter, Empty, Repeat, Sink,
 };
 
 // Re-export io::Error so that users don't have to deal