feat(rt): make tokio runtime optional

A Cargo feature `runtime` is added, which is enabled by default, that
includes the following:

- The `client::HttpConnector`, which uses `tokio::net::TcpStream`.
- The `server::AddrStream`, which uses `tokio::net::TcpListener`.
- The `hyper::rt` module, which includes useful utilities to work with
  the runtime without needing to import `futures` or `tokio` explicity.

Disabling the feature removes many of these niceties, but allows people
to use hyper in environments that have an alternative runtime, without
needing to download an unused one.

src/error.rs

@@ -39,6 +39,7 @@ pub(crate) enum Kind {
     /// Error occurred while connecting.
     Connect,
     /// Error creating a TcpListener.
+    #[cfg(feature = "runtime")]
     Listen,
     /// Error accepting on an Incoming stream.
     Accept,
@@ -171,6 +172,7 @@ impl Error {
         Error::new(Kind::Io, Some(cause.into()))
     }
 
+    #[cfg(feature = "runtime")]
     pub(crate) fn new_listen<E: Into<Cause>>(cause: E) -> Error {
         Error::new(Kind::Listen, Some(cause.into()))
     }
@@ -258,6 +260,7 @@ impl StdError for Error {
             Kind::Closed => "connection closed",
             Kind::Connect => "an error occurred trying to connect",
             Kind::Canceled => "an operation was canceled internally before starting",
+            #[cfg(feature = "runtime")]
             Kind::Listen => "error creating server listener",
             Kind::Accept => "error accepting connection",
             Kind::NewService => "calling user's new_service failed",