routes macro allowing multiple paths per handler (#2718)

* WIP: basic implementation for `routes` macro

* chore: changelog, docs, tests

* error on missing methods

* Apply suggestions from code review

Co-authored-by: Igor Aleksanov <popzxc@yandex.ru>

* update test stderr expectation

* add additional tests

* fix stderr output

* remove useless ResourceType

this is dead code from back when .to and .to_async were different ways to add a service

Co-authored-by: Igor Aleksanov <popzxc@yandex.ru>
Co-authored-by: Rob Ede <robjtede@icloud.com>

actix-web-codegen/tests/trybuild/routes-ok.rs

@@ -0,0 +1,23 @@
+use actix_web_codegen::*;
+
+#[routes]
+#[get("/")]
+#[post("/")]
+async fn index() -> String {
+    "Hello World!".to_owned()
+}
+
+#[actix_web::main]
+async fn main() {
+    use actix_web::App;
+
+    let srv = actix_test::start(|| App::new().service(index));
+
+    let request = srv.get("/");
+    let response = request.send().await.unwrap();
+    assert!(response.status().is_success());
+
+    let request = srv.post("/");
+    let response = request.send().await.unwrap();
+    assert!(response.status().is_success());
+}