Implement multibody joints and the new solver

examples3d/debug_rollback3.rs

@@ -7,7 +7,9 @@ pub fn init_world(testbed: &mut Testbed) {
      */
     let mut bodies = RigidBodySet::new();
     let mut colliders = ColliderSet::new();
-    let joints = JointSet::new();
+    let impulse_joints = ImpulseJointSet::new();
+    let multibody_joints = MultibodyJointSet::new();
+
     /*
      * Ground.
      */
@@ -65,6 +67,6 @@ pub fn init_world(testbed: &mut Testbed) {
     /*
      * Set up the testbed.
      */
-    testbed.set_world(bodies, colliders, joints);
+    testbed.set_world(bodies, colliders, impulse_joints, multibody_joints);
     testbed.look_at(point![10.0, 10.0, 10.0], Point::origin());
 }