Impl Default for a few structs

src/dynamics/rigid_body_set.rs

@@ -28,7 +28,7 @@ impl BodyPair {
 }
 
 #[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
-#[derive(Clone)]
+#[derive(Clone, Default)]
 /// A set of rigid bodies that can be handled by a physics pipeline.
 pub struct RigidBodySet {
     // NOTE: the pub(crate) are needed by the broad phase