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.

src/error.rs

@@ -203,8 +203,8 @@ impl Error {
         Error::new(Kind::UnsupportedRequestMethod, None)
     }
 
-    pub(crate) fn new_user_new_service(err: io::Error) -> Error {
-        Error::new(Kind::NewService, Some(Box::new(err)))
+    pub(crate) fn new_user_new_service<E: Into<Cause>>(cause: E) -> Error {
+        Error::new(Kind::NewService, Some(cause.into()))
     }
 
     pub(crate) fn new_user_service<E: Into<Cause>>(cause: E) -> Error {