feat(service): introduce hyper-specific Service

This introduces the `hyper::service` module, which replaces
`tokio-service`.

Since the trait is specific to hyper, its associated
types have been adjusted. It didn't make sense to need to define
`Service<Request=http::Request>`, since we already know the context is
HTTP. Instead, the request and response bodies are associated types now,
and slightly stricter bounds have been placed on `Error`.

The helpers `service_fn` and `service_fn_ok` should be sufficient for
now to ease creating `Service`s.

The `NewService` trait now allows service creation to also be
asynchronous.

These traits are similar to `tower` in nature, and possibly will be
replaced completely by it in the future. For now, hyper defining its own
allows the traits to have better context, and prevents breaking changes
in `tower` from affecting hyper.

Closes #1461

BREAKING CHANGE: The `Service` trait has changed: it has some changed
  associated types, and `call` is now bound to `&mut self`.

  The `NewService` trait has changed: it has some changed associated
  types, and `new_service` now returns a `Future`.

  `Client` no longer implements `Service` for now.

  `hyper::server::conn::Serve` now returns `Connecting` instead of
  `Connection`s, since `new_service` can now return a `Future`. The
  `Connecting` is a future wrapping the new service future, returning
  a `Connection` afterwards. In many cases, `Future::flatten` can be
  used.

examples/hello.rs

@@ -6,8 +6,8 @@ extern crate tokio;
 
 use futures::Future;
 
-use hyper::{Body, Response};
-use hyper::server::{Server, const_service, service_fn};
+use hyper::{Body, Response, Server};
+use hyper::service::service_fn_ok;
 
 static PHRASE: &'static [u8] = b"Hello World!";
 
@@ -16,10 +16,16 @@ fn main() {
 
     let addr = ([127, 0, 0, 1], 3000).into();
 
-    let new_service = const_service(service_fn(|_| {
-        //TODO: when `!` is stable, replace error type
-        Ok::<_, hyper::Error>(Response::new(Body::from(PHRASE)))
-    }));
+    // new_service is run for each connection, creating a 'service'
+    // to handle requests for that specific connection.
+    let new_service = || {
+        // This is the `Service` that will handle the connection.
+        // `service_fn_ok` is a helper to convert a function that
+        // returns a Response into a `Service`.
+        service_fn_ok(|_| {
+            Response::new(Body::from(PHRASE))
+        })
+    };
 
     let server = Server::bind(&addr)
         .serve(new_service)