chore: Fix examples not working with cargo run (#998)

* chore: Fix examples not working with `cargo run`

## Motivation

PR #991 moved the `tokio` crate to its own subdirectory, but did not
move the `examples` directory into `tokio/examples`. While attempting to
use the examples for testing another change, I noticed that #991 had
broken the ability to use `cargo run`, as the examples were no longer
considered part of a crate that cargo was aware of:

```
tokio on master [$] via 🦀v1.33.0 at ☸️ aks-eliza-dev
➜  cargo run --example chat
error: no example target named `chat`

Did you mean `echo`?
```

## Solution

This branch moves the examples into the `tokio` directory, so cargo is
now once again aware of them:

```
tokio on eliza/fix-examples [$] via 🦀v1.33.0 at ☸️ aks-eliza-dev
➜  cargo run --example chat
   Compiling tokio-executor v0.1.7 (/Users/eliza/Code/tokio/tokio-executor)
   Compiling tokio-reactor v0.1.9
   Compiling tokio-threadpool v0.1.13
   Compiling tokio-current-thread v0.1.6
   Compiling tokio-timer v0.2.10
   Compiling tokio-uds v0.2.5
   Compiling tokio-udp v0.1.3
   Compiling tokio-tcp v0.1.3
   Compiling tokio-fs v0.1.6
   Compiling tokio v0.1.18 (/Users/eliza/Code/tokio/tokio)
    Finished dev [unoptimized + debuginfo] target(s) in 7.04s
     Running `target/debug/examples/chat`
server running on localhost:6142
```

Signed-off-by: Eliza Weisman <[email protected]>

Signed-off-by: Eliza Weisman <[email protected]>

examples/echo-udp.rs

@@ -1,74 +0,0 @@
-//! 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!
-
-#![deny(warnings)]
-
-#[macro_use]
-extern crate futures;
-extern crate tokio;
-
-use std::net::SocketAddr;
-use std::{env, io};
-
-use tokio::net::UdpSocket;
-use tokio::prelude::*;
-
-struct Server {
-    socket: UdpSocket,
-    buf: Vec<u8>,
-    to_send: Option<(usize, SocketAddr)>,
-}
-
-impl Future for Server {
-    type Item = ();
-    type Error = io::Error;
-
-    fn poll(&mut self) -> Poll<(), io::Error> {
-        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)) = self.to_send {
-                let amt = try_ready!(self.socket.poll_send_to(&self.buf[..size], &peer));
-                println!("Echoed {}/{} bytes to {}", amt, size, peer);
-                self.to_send = None;
-            }
-
-            // If we're here then `to_send` is `None`, so we take a look for the
-            // next message we're going to echo back.
-            self.to_send = Some(try_ready!(self.socket.poll_recv_from(&mut self.buf)));
-        }
-    }
-}
-
-fn main() -> Result<(), Box<std::error::Error>> {
-    let addr = env::args().nth(1).unwrap_or("127.0.0.1:8080".to_string());
-    let addr = addr.parse::<SocketAddr>()?;
-
-    let socket = UdpSocket::bind(&addr)?;
-    println!("Listening on: {}", socket.local_addr()?);
-
-    let server = Server {
-        socket: socket,
-        buf: vec![0; 1024],
-        to_send: None,
-    };
-
-    // This starts the server task.
-    //
-    // `map_err` handles the error by logging it and maps the future to a type
-    // that can be spawned.
-    //
-    // `tokio::run` spawns the task on the Tokio runtime and starts running.
-    tokio::run(server.map_err(|e| println!("server error = {:?}", e)));
-    Ok(())
-}