Refactor the parallel solver code the same way we did with the non-parallel solver.

src/dynamics/solver/velocity_constraint.rs

@@ -132,8 +132,8 @@ pub(crate) struct VelocityConstraint {
 impl VelocityConstraint {
     #[cfg(feature = "parallel")]
     pub fn num_active_constraints(manifold: &ContactManifold) -> usize {
-        let rest = manifold.num_active_contacts() % MAX_MANIFOLD_POINTS != 0;
-        manifold.num_active_contacts() / MAX_MANIFOLD_POINTS + rest as usize
+        let rest = manifold.data.solver_contacts.len() % MAX_MANIFOLD_POINTS != 0;
+        manifold.data.solver_contacts.len() / MAX_MANIFOLD_POINTS + rest as usize
     }
 
     pub fn generate(