Move all examples to their own crates (#201)

This makes it much clearer which dependencies each example has.

examples/versioning/src/main.rs

@@ -0,0 +1,74 @@
+//! Run with
+//!
+//! ```not_rust
+//! cargo run -p example-versioning
+//! ```
+
+use axum::{
+    async_trait,
+    body::{Bytes, Full},
+    extract::{FromRequest, Path, RequestParts},
+    handler::get,
+    http::{Response, StatusCode},
+    response::IntoResponse,
+    route,
+    routing::RoutingDsl,
+};
+use std::collections::HashMap;
+use std::net::SocketAddr;
+
+#[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_versioning=debug")
+    }
+    tracing_subscriber::fmt::init();
+
+    // build our application with some routes
+    let app = route("/:version/foo", get(handler));
+
+    // 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(version: Version) {
+    println!("received request with version {:?}", version);
+}
+
+#[derive(Debug)]
+enum Version {
+    V1,
+    V2,
+    V3,
+}
+
+#[async_trait]
+impl<B> FromRequest<B> for Version
+where
+    B: Send,
+{
+    type Rejection = Response<Full<Bytes>>;
+
+    async fn from_request(req: &mut RequestParts<B>) -> Result<Self, Self::Rejection> {
+        let params = Path::<HashMap<String, String>>::from_request(req)
+            .await
+            .map_err(IntoResponse::into_response)?;
+
+        let version = params
+            .get("version")
+            .ok_or_else(|| (StatusCode::NOT_FOUND, "version param missing").into_response())?;
+
+        match version.as_str() {
+            "v1" => Ok(Version::V1),
+            "v2" => Ok(Version::V2),
+            "v3" => Ok(Version::V3),
+            _ => Err((StatusCode::NOT_FOUND, "unknown version").into_response()),
+        }
+    }
+}