examples: add a simple server example

This commit adds an example *server* that is roughly contemporary with
the existing "simpleclient".

It is the absolute bare minimum needed to run a server using Rustls
(e.g. it only accepts a single connection before terminating).

You can run the server with:
```
CERTFILE=test-ca/rsa/end.fullchain PRIV_KEY_FILE=test-ca/rsa/end.key  cargo run --package rustls-examples --bin simpleserver
```

And connect to it with a client:
```
cargo run --package rustls-examples --bin tlsclient-mio -- --port 4443 --cafile test-ca/rsa/ca.cert localhost --http
```

examples/src/bin/simpleserver.rs

@@ -0,0 +1,51 @@
+//! This is the simplest possible server using rustls that does something useful:
+//! it accepts the default configuration, loads a server certificate and private key,
+//! and then accepts a single client connection.
+//!
+//! You must either set the CERTFILE and PRIV_KEY_FILE env vars to point to a server
+//! certificate and private key, or place 'localhost.pem' and 'localhost-key.pem' in
+//! the directory you run this example from.
+//!
+//! Note that `unwrap()` is used to deal with networking errors; this is not something
+//! that is sensible outside of example code.
+
+use std::error::Error as StdError;
+use std::fs::File;
+use std::io::{BufReader, Read, Write};
+use std::net::TcpListener;
+use std::sync::Arc;
+
+const CERTFILE: &str = match option_env!("CERTFILE") {
+    Some(certfile) => certfile,
+    None => "localhost.pem",
+};
+
+const PRIV_KEY_FILE: &str = match option_env!("PRIV_KEY_FILE") {
+    Some(priv_key_file) => priv_key_file,
+    None => "localhost-key.pem",
+};
+
+fn main() -> Result<(), Box<dyn StdError>> {
+    let certs = rustls_pemfile::certs(&mut BufReader::new(&mut File::open(CERTFILE)?))
+        .collect::<Result<Vec<_>, _>>()?;
+    let private_key =
+        rustls_pemfile::private_key(&mut BufReader::new(&mut File::open(PRIV_KEY_FILE)?))?.unwrap();
+    let config = rustls::ServerConfig::builder()
+        .with_no_client_auth()
+        .with_single_cert(certs, private_key)?;
+
+    let listener = TcpListener::bind(format!("[::]:{}", 4443)).unwrap();
+    let (mut stream, _) = listener.accept()?;
+
+    let mut conn = rustls::ServerConnection::new(Arc::new(config))?;
+    conn.complete_io(&mut stream)?;
+
+    conn.writer()
+        .write_all(b"Hello from the server")?;
+    conn.complete_io(&mut stream)?;
+    let mut buf = [0; 64];
+    let len = conn.reader().read(&mut buf)?;
+    println!("Received message from client: {:?}", &buf[..len]);
+
+    Ok(())
+}