io: move into tokio crate (#1691)

A step towards collapsing Tokio sub crates into a single `tokio`
crate (#1318).

The `io` implementation is now provided by the main `tokio` crate.
Functionality can be opted out of by using the various net related
feature flags.

tokio-util/src/codec/decoder.rs

@@ -1,7 +1,7 @@
 use crate::codec::encoder::Encoder;
 use crate::codec::Framed;
 
-use tokio_io::{AsyncRead, AsyncWrite};
+use tokio::io::{AsyncRead, AsyncWrite};
 
 use bytes::BytesMut;
 use std::io;