Fix clippy warnings

Add default implementations
Return &mut View instead of &mut Box<View>

src/views/list_view.rs

@@ -31,9 +31,9 @@ impl ListChild {
         }
     }
 
-    fn view(&mut self) -> Option<&mut Box<View>> {
+    fn view(&mut self) -> Option<&mut View> {
         match *self {
-            ListChild::Row(_, ref mut view) => Some(view),
+            ListChild::Row(_, ref mut view) => Some(view.as_mut()),
             _ => None,
         }
     }

src/views/radio.rs

@@ -32,6 +32,12 @@ pub struct RadioGroup<T> {
     state: Rc<RefCell<SharedState<T>>>,
 }
 
+impl <T> Default for RadioGroup<T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl<T> RadioGroup<T> {
     /// Creates an empty group for radio buttons.
     pub fn new() -> Self {

src/views/select_view.rs

@@ -67,6 +67,12 @@ pub struct SelectView<T = String> {
     last_size: Vec2,
 }
 
+impl <T: 'static> Default for SelectView<T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl<T: 'static> SelectView<T> {
     /// Creates a new empty SelectView.
     pub fn new() -> Self {