Wrap the temporarily stored user callback in a type to enforce

invariants

This also removes the need for "box"ing the callback in favour of
storing a raw `*mut` pointer. We can do this by ensuring that we never
store the pointer for longer than the lifetime of the user callback,
which is the duration of a call to `poll_events` or `run_forever`.

Also removes old commented out event code from the window module.

tests/events_loop.rs

@@ -0,0 +1,11 @@
+extern crate winit;
+
+// A part of the API requirement for `EventsLoop` is that it is `Send` + `Sync`.
+//
+// This short test will only compile if the `EventsLoop` is `Send` + `Sync`. 
+#[test]
+fn send_sync() {
+    fn check_send_sync<T: Send + Sync>(_: T) {}
+    let events_loop = winit::EventsLoop::new();
+    check_send_sync(events_loop);
+}