Move all examples to their own crates (#201)

This makes it much clearer which dependencies each example has.

examples/global-404-handler/src/main.rs

@@ -0,0 +1,55 @@
+//! Run with
+//!
+//! ```not_rust
+//! cargo run -p example-global-404-handler
+//! ```
+
+use axum::{
+    body::{box_body, Body, BoxBody},
+    handler::get,
+    http::{Response, StatusCode},
+    response::Html,
+    route,
+    routing::RoutingDsl,
+};
+use std::net::SocketAddr;
+use tower::util::MapResponseLayer;
+
+#[tokio::main]
+async fn main() {
+    // Set the RUST_LOG, if it hasn't been explicitly defined
+    if std::env::var("RUST_LOG").is_err() {
+        std::env::set_var("RUST_LOG", "example_global_404_handler=debug")
+    }
+    tracing_subscriber::fmt::init();
+
+    // build our application with a route
+    let app = route("/", get(handler))
+        // make sure this is added as the very last thing
+        .layer(MapResponseLayer::new(map_404));
+
+    // run it
+    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
+    tracing::debug!("listening on {}", addr);
+    axum::Server::bind(&addr)
+        .serve(app.into_make_service())
+        .await
+        .unwrap();
+}
+
+async fn handler() -> Html<&'static str> {
+    Html("<h1>Hello, World!</h1>")
+}
+
+fn map_404(response: Response<BoxBody>) -> Response<BoxBody> {
+    if response.status() == StatusCode::NOT_FOUND
+        || response.status() == StatusCode::METHOD_NOT_ALLOWED
+    {
+        return Response::builder()
+            .status(StatusCode::NOT_FOUND)
+            .body(box_body(Body::from("nothing to see here")))
+            .unwrap();
+    }
+
+    response
+}