Reorganize the event_loop module

Split it up into a number of targeted modules for each purpose, for example loop
data, I/O sources, timeouts, and channels. No actual change is intended to be
part of this commit.

src/lib.rs

@@ -27,7 +27,6 @@ mod slot;
 #[path = "../../src/lock.rs"]
 mod lock;
 mod mpsc_queue;
-mod channel;
 
 pub use event_loop::{Loop, LoopPin, LoopHandle, AddSource, AddTimeout};
 pub use event_loop::{LoopData, AddLoopData, TimeoutToken, IoToken};