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/hello_world.rs

@@ -0,0 +1,33 @@
+//! Hello world server.
+//!
+//! A simple client that opens a TCP stream, writes "hello world\n", and closes
+//! the connection.
+//!
+//! You can test this out by running:
+//!
+//!     ncat -l 6142
+//!
+//! And then in another terminal run:
+//!
+//!     cargo run --example hello_world
+
+#![warn(rust_2018_idioms)]
+
+use tokio::io::AsyncWriteExt;
+use tokio::net::TcpStream;
+
+use std::error::Error;
+
+#[tokio::main]
+pub async fn main() -> Result<(), Box<dyn Error>> {
+    // Open a TCP stream to the socket address.
+    //
+    // Note that this is the Tokio TcpStream, which is fully async.
+    let mut stream = TcpStream::connect("127.0.0.1:6142").await?;
+    println!("created stream");
+
+    let result = stream.write(b"hello world\n").await;
+    println!("wrote to stream; success={:?}", result.is_ok());
+
+    Ok(())
+}