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_testbed/lib.rs

@@ -1,12 +1,4 @@
 extern crate nalgebra as na;
-#[cfg(feature = "dim2")]
-extern crate parry2d as parry;
-#[cfg(feature = "dim3")]
-extern crate parry3d as parry;
-#[cfg(feature = "dim2")]
-extern crate rapier2d as rapier;
-#[cfg(feature = "dim3")]
-extern crate rapier3d as rapier;
 
 #[macro_use]
 extern crate bitflags;