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`.

src/error.rs

@@ -171,12 +171,12 @@ impl Error {
         Error::new(Kind::Io, Some(cause.into()))
     }
 
-    pub(crate) fn new_listen(err: io::Error) -> Error {
-        Error::new(Kind::Listen, Some(err.into()))
+    pub(crate) fn new_listen<E: Into<Cause>>(cause: E) -> Error {
+        Error::new(Kind::Listen, Some(cause.into()))
     }
 
-    pub(crate) fn new_accept(err: io::Error) -> Error {
-        Error::new(Kind::Accept, Some(Box::new(err)))
+    pub(crate) fn new_accept<E: Into<Cause>>(cause: E) -> Error {
+        Error::new(Kind::Accept, Some(cause.into()))
     }
 
     pub(crate) fn new_connect<E: Into<Cause>>(cause: E) -> Error {