feat(lib): redesign API to use Futures and Tokio

There are many changes involved with this, but let's just talk about
user-facing changes.

- Creating a `Client` and `Server` now needs a Tokio `Core` event loop
to attach to.
- `Request` and `Response` both no longer implement the
`std::io::{Read,Write}` traits, but instead represent their bodies as a
`futures::Stream` of items, where each item is a `Chunk`.
- The `Client.request` method now takes a `Request`, instead of being
used as a builder, and returns a `Future` that resolves to `Response`.
- The `Handler` trait for servers is no more, and instead the Tokio
`Service` trait is used. This allows interoperability with generic
middleware.

BREAKING CHANGE: A big sweeping set of breaking changes.

src/version.rs

@@ -19,6 +19,8 @@ pub enum HttpVersion {
     H2,
     /// `HTTP/2.0` over cleartext
     H2c,
+    #[doc(hidden)]
+    __DontMatchMe,
 }
 
 impl fmt::Display for HttpVersion {
@@ -29,6 +31,7 @@ impl fmt::Display for HttpVersion {
             Http11 => "HTTP/1.1",
             H2 => "h2",
             H2c => "h2c",
+            HttpVersion::__DontMatchMe => unreachable!(),
         })
     }
 }