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_link.rs

@@ -5,6 +5,8 @@ use crate::math::{Isometry, Real, Vector};
 use crate::prelude::RigidBodyVelocity;
 
 /// One link of a multibody.
+#[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
+#[derive(Clone)]
 pub struct MultibodyLink {
     // FIXME: make all those private.
     pub(crate) internal_id: usize,
@@ -96,6 +98,8 @@ impl MultibodyLink {
 }
 
 // FIXME: keep this even if we already have the Index2 traits?
+#[cfg_attr(feature = "serde-serialize", derive(Serialize, Deserialize))]
+#[derive(Clone)]
 pub(crate) struct MultibodyLinkVec(pub Vec<MultibodyLink>);
 
 impl MultibodyLinkVec {