Run clippy on CI

Fixes #1402.

src/event_loop.rs

@@ -94,6 +94,8 @@ impl<T> EventLoopBuilder<T> {
     ///   will fall back on X11. If this variable is set with any other value, winit will panic.
     #[inline]
     pub fn build(&mut self) -> EventLoop<T> {
+        // Certain platforms accept a mutable reference in their API.
+        #[allow(clippy::unnecessary_mut_passed)]
         EventLoop {
             event_loop: platform_impl::EventLoop::new(&mut self.platform_specific),
             _marker: PhantomData,
@@ -221,6 +223,12 @@ impl EventLoop<()> {
     }
 }
 
+impl Default for EventLoop<()> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl<T> EventLoop<T> {
     #[deprecated = "Use `EventLoopBuilder::<T>::with_user_event().build()` instead."]
     pub fn with_user_event() -> EventLoop<T> {