Update to futures master

* Remove `LoopData` as it's no longer necessary
* Add `LoopHandle::spawn` to spawn new futures onto an event loop
* Add `LoopData::spawn` to also spawn new futures onto an event loop
* Rejigger the implementation of the event loop a bit (make a slab of futures),
  but otherwise everything else is pretty constant.

src/lib.rs

@@ -32,7 +32,7 @@ mod udp;
 
 pub use channel::{Sender, Receiver};
 pub use event_loop::{Loop, LoopPin, LoopHandle, AddSource, AddTimeout};
-pub use event_loop::{LoopData, AddLoopData, TimeoutToken, IoToken};
+pub use event_loop::{TimeoutToken, IoToken};
 pub use readiness_stream::ReadinessStream;
 pub use tcp::{TcpListener, TcpStream};
 pub use timeout::Timeout;