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/common/mod.rs

@@ -1,6 +1,5 @@
 mod exec;
+mod never;
 
 pub(crate) use self::exec::Exec;
-
-#[derive(Debug)]
-pub enum Never {}
+pub use self::never::Never;

src/common/never.rs

@@ -0,0 +1,22 @@
+//! An uninhabitable type meaning it can never happen.
+//!
+//! To be replaced with `!` once it is stable.
+
+use std::error::Error;
+use std::fmt;
+
+#[derive(Debug)]
+pub enum Never {}
+
+impl fmt::Display for Never {
+    fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
+        match *self {}
+    }
+}
+
+impl Error for Never {
+    fn description(&self) -> &str {
+        match *self {}
+    }
+}
+