feat(server): re-design Server as higher-level API

The `hyper::Server` is now a proper higher-level API for running HTTP
servers. There is a related `hyper::server::Builder` type, to construct
a `Server`. All other types (`Http`, `Serve`, etc) were moved into the
"lower-level" `hyper::server::conn` module.

The `Server` is a `Future` representing a listening HTTP server. Options
needed to build one are set on the `Builder`.

As `Server` is just a `Future`, it no longer owns a thread-blocking
executor, and can thus be run next to other servers, clients, or
what-have-you.

Closes #1322
Closes #1263

BREAKING CHANGE: The `Server` is no longer created from `Http::bind`,
  nor is it `run`. It is a `Future` that must be polled by an
  `Executor`.

  The `hyper::server::Http` type has move to
  `hyper::server::conn::Http`.

tests/support/mod.rs

@@ -123,9 +123,11 @@ macro_rules! __internal_req_res_prop {
     );
     (headers: $map:tt) => ({
         #[allow(unused_mut)]
+        {
         let mut headers = HeaderMap::new();
         __internal_headers!(headers, $map);
         headers
+        }
     });
     ($prop_name:ident: $prop_val:expr) => (
         From::from($prop_val)
@@ -228,7 +230,7 @@ pub fn __run_test(cfg: __TestConfig) {
     });
     let new_service = hyper::server::const_service(service);
 
-    let serve = hyper::server::Http::new()
+    let serve = hyper::server::conn::Http::new()
         .http2_only(cfg.server_version == 2)
         .executor(rt.executor())
         .serve_addr_handle(
@@ -244,7 +246,7 @@ pub fn __run_test(cfg: __TestConfig) {
     let (success_tx, success_rx) = oneshot::channel();
     let expected_connections = cfg.connections;
     let server = serve
-        .fold(0, move |cnt, conn: hyper::server::Connection<_, _>| {
+        .fold(0, move |cnt, conn| {
             exe.spawn(conn.map_err(|e| panic!("server connection error: {}", e)));
             Ok::<_, hyper::Error>(cnt + 1)
         })