Move all examples to their own crates (#201)

This makes it much clearer which dependencies each example has.

examples/hello-world/src/main.rs

@@ -0,0 +1,26 @@
+//! Run with
+//!
+//! ```not_rust
+//! cargo run -p example-hello-world
+//! ```
+
+use axum::{handler::get, route, routing::RoutingDsl};
+use std::net::SocketAddr;
+
+#[tokio::main]
+async fn main() {
+    // build our application with a route
+    let app = route("/foo", get(handler));
+
+    // run it
+    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
+    println!("listening on {}", addr);
+    axum::Server::bind(&addr)
+        .serve(app.into_make_service())
+        .await
+        .unwrap();
+}
+
+async fn handler() -> &'static str {
+    "<h1>Hello, World!</h1>"
+}