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/udp-client.rs

@@ -0,0 +1,72 @@
+//! A UDP client that just sends everything it gets via `stdio` in a single datagram, and then
+//! waits for a reply.
+//!
+//! For the reasons of simplicity data from `stdio` is read until `EOF` in a blocking manner.
+//!
+//! You can test this out by running an echo server:
+//!
+//! ```
+//!     $ cargo run --example echo-udp -- 127.0.0.1:8080
+//! ```
+//!
+//! and running the client in another terminal:
+//!
+//! ```
+//!     $ cargo run --example udp-client
+//! ```
+//!
+//! You can optionally provide any custom endpoint address for the client:
+//!
+//! ```
+//!     $ cargo run --example udp-client -- 127.0.0.1:8080
+//! ```
+//!
+//! Don't forget to pass `EOF` to the standard input of the client!
+//!
+//! Please mind that since the UDP protocol doesn't have any capabilities to detect a broken
+//! connection the server needs to be run first, otherwise the client will block forever.
+
+#![warn(rust_2018_idioms)]
+
+use std::env;
+use std::error::Error;
+use std::io::{stdin, Read};
+use std::net::SocketAddr;
+use tokio::net::UdpSocket;
+
+fn get_stdin_data() -> Result<Vec<u8>, Box<dyn std::error::Error>> {
+    let mut buf = Vec::new();
+    stdin().read_to_end(&mut buf)?;
+    Ok(buf)
+}
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn Error>> {
+    let remote_addr: SocketAddr = env::args()
+        .nth(1)
+        .unwrap_or("127.0.0.1:8080".into())
+        .parse()?;
+
+    // We use port 0 to let the operating system allocate an available port for us.
+    let local_addr: SocketAddr = if remote_addr.is_ipv4() {
+        "0.0.0.0:0"
+    } else {
+        "[::]:0"
+    }
+    .parse()?;
+
+    let mut socket = UdpSocket::bind(local_addr).await?;
+    const MAX_DATAGRAM_SIZE: usize = 65_507;
+    socket.connect(&remote_addr).await?;
+    let data = get_stdin_data()?;
+    socket.send(&data).await?;
+    let mut data = vec![0u8; MAX_DATAGRAM_SIZE];
+    let len = socket.recv(&mut data).await?;
+    println!(
+        "Received {} bytes:\n{}",
+        len,
+        String::from_utf8_lossy(&data[..len])
+    );
+
+    Ok(())
+}