Fix some solver issues

- Fix the wrong codepath taken  by the solver for contacts involving a collider without parent.
- Properly adress the non-linear treatment of the friction direction
- Simplify the sleeping strategy
- Add an impulse resolution multiplier

src/dynamics/joint/multibody_joint/multibody.rs

@@ -62,7 +62,10 @@ fn concat_rb_mass_matrix(
 }
 
 /// An articulated body simulated using the reduced-coordinates approach.
+#[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
+#[derive(Clone)]
 pub struct Multibody {
+    // TODO: serialization: skip the workspace fields.
     links: MultibodyLinkVec,
     pub(crate) velocities: DVector<Real>,
     pub(crate) damping: DVector<Real>,