codec: move into tokio-util (#1675)

Related to #1318, Tokio APIs that are "less stable" are moved into a new
`tokio-util` crate. This crate will mirror `tokio` and provide
additional APIs that may require a greater rate of breaking changes.

As examples require `tokio-util`, they are moved into a separate
crate (`examples`). This has the added advantage of being able to avoid
example only dependencies in the `tokio` crate.

examples/echo-udp.rs

@@ -0,0 +1,69 @@
+//! An UDP echo server that just sends back everything that it receives.
+//!
+//! If you're on Unix you can test this out by in one terminal executing:
+//!
+//!     cargo run --example echo-udp
+//!
+//! and in another terminal you can run:
+//!
+//!     cargo run --example connect -- --udp 127.0.0.1:8080
+//!
+//! Each line you type in to the `nc` terminal should be echo'd back to you!
+
+#![warn(rust_2018_idioms)]
+
+use std::error::Error;
+use std::net::SocketAddr;
+use std::{env, io};
+use tokio;
+use tokio::net::UdpSocket;
+
+struct Server {
+    socket: UdpSocket,
+    buf: Vec<u8>,
+    to_send: Option<(usize, SocketAddr)>,
+}
+
+impl Server {
+    async fn run(self) -> Result<(), io::Error> {
+        let Server {
+            mut socket,
+            mut buf,
+            mut to_send,
+        } = self;
+
+        loop {
+            // First we check to see if there's a message we need to echo back.
+            // If so then we try to send it back to the original source, waiting
+            // until it's writable and we're able to do so.
+            if let Some((size, peer)) = to_send {
+                let amt = socket.send_to(&buf[..size], &peer).await?;
+
+                println!("Echoed {}/{} bytes to {}", amt, size, peer);
+            }
+
+            // If we're here then `to_send` is `None`, so we take a look for the
+            // next message we're going to echo back.
+            to_send = Some(socket.recv_from(&mut buf).await?);
+        }
+    }
+}
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn Error>> {
+    let addr = env::args().nth(1).unwrap_or("127.0.0.1:8080".to_string());
+
+    let socket = UdpSocket::bind(&addr).await?;
+    println!("Listening on: {}", socket.local_addr()?);
+
+    let server = Server {
+        socket,
+        buf: vec![0; 1024],
+        to_send: None,
+    };
+
+    // This starts the server task.
+    server.run().await?;
+
+    Ok(())
+}