feat(server): add const_service and service_fn helpers

- `const_service` creates a `NewService` that clones references to the
  wrapped service.
- `service_fn` creates a `Service` from a function. Useful with closures.

examples/hello.rs

@@ -3,35 +3,23 @@ extern crate hyper;
 extern crate futures;
 extern crate pretty_env_logger;
 
-use futures::future::FutureResult;
-
 use hyper::header::{ContentLength, ContentType};
-use hyper::server::{Http, Service, Request, Response};
+use hyper::server::{Http, Response, const_service, service_fn};
 
 static PHRASE: &'static [u8] = b"Hello World!";
 
-struct Hello;
-
-impl Service for Hello {
-    type Request = Request;
-    type Response = Response;
-    type Error = hyper::Error;
-    type Future = FutureResult<Response, hyper::Error>;
-    fn call(&self, _req: Request) -> Self::Future {
-        futures::future::ok(
-            Response::new()
-                .with_header(ContentLength(PHRASE.len() as u64))
-                .with_header(ContentType::plaintext())
-                .with_body(PHRASE)
-        )
-    }
-
-}
-
 fn main() {
     pretty_env_logger::init().unwrap();
-    let addr = "127.0.0.1:3000".parse().unwrap();
-    let mut server = Http::new().bind(&addr, || Ok(Hello)).unwrap();
+    let addr = ([127, 0, 0, 1], 3000).into();
+
+    let new_service = const_service(service_fn(|_| {
+        Ok(Response::<hyper::Body>::new()
+            .with_header(ContentLength(PHRASE.len() as u64))
+            .with_header(ContentType::plaintext())
+            .with_body(PHRASE))
+    }));
+
+    let mut server = Http::new().bind(&addr, new_service).unwrap();
     server.no_proto();
     println!("Listening on http://{} with 1 thread.", server.local_addr().unwrap());
     server.run().unwrap();