Move all examples to their own crates (#201)

This makes it much clearer which dependencies each example has.

examples/sse/src/main.rs

@@ -0,0 +1,62 @@
+//! Run with
+//!
+//! ```not_rust
+//! cargo run -p example-sse
+//! ```
+
+use axum::{
+    extract::TypedHeader,
+    handler::get,
+    http::StatusCode,
+    response::sse::{sse, Event, Sse},
+    routing::{nest, RoutingDsl},
+};
+use futures::stream::{self, Stream};
+use std::{convert::Infallible, net::SocketAddr, time::Duration};
+use tokio_stream::StreamExt as _;
+use tower_http::{services::ServeDir, trace::TraceLayer};
+
+#[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_sse=debug,tower_http=debug")
+    }
+    tracing_subscriber::fmt::init();
+
+    let static_files_service = axum::service::get(
+        ServeDir::new("examples/sse/assets").append_index_html_on_directories(true),
+    )
+    .handle_error(|error: std::io::Error| {
+        Ok::<_, std::convert::Infallible>((
+            StatusCode::INTERNAL_SERVER_ERROR,
+            format!("Unhandled internal error: {}", error),
+        ))
+    });
+
+    // build our application with a route
+    let app = nest("/", static_files_service)
+        .route("/sse", get(sse_handler))
+        .layer(TraceLayer::new_for_http());
+
+    // 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 sse_handler(
+    TypedHeader(user_agent): TypedHeader<headers::UserAgent>,
+) -> Sse<impl Stream<Item = Result<Event, Infallible>>> {
+    println!("`{}` connected", user_agent.as_str());
+
+    // A `Stream` that repeats an event every second
+    let stream = stream::repeat_with(|| Event::default().data("hi!"))
+        .map(Ok)
+        .throttle(Duration::from_secs(1));
+
+    sse(stream)
+}