Formalize thread-safety guarantees (#322)

tests/send_objects.rs

@@ -0,0 +1,23 @@
+extern crate winit;
+
+fn needs_send<T:Send>() {}
+
+#[test]
+fn events_loop_proxy_send() {
+    // ensures that `winit::EventsLoopProxy` implements `Send`
+    needs_send::<winit::EventsLoopProxy>();
+}
+
+#[test]
+fn window_send() {
+    // ensures that `winit::Window` implements `Send`
+    needs_send::<winit::Window>();
+}
+
+#[test]
+fn ids_send() {
+    // ensures that the various `..Id` types implement `Send`
+    needs_send::<winit::WindowId>();
+    needs_send::<winit::DeviceId>();
+    needs_send::<winit::MonitorId>();
+}