feat: persistent islands + manifold reduction (#895)

* feat: initial implementation of contact manifold reduction

* feat: try bepu-like manifold reduction

* feat: simplification of the constraints counting and indexing logic

* feat: add concept of incremental islands with a single awake island

More islands manager fixes

* feat: start adding support for multiple awake islands

* feat: add more timings

* feat: implement incremental island split & merge

* chore: refactor islands manager into multiple files

* chore: refactor manifold reduction to its own file + add naive reduction method

* feat: add islands manager validation checks

* fix various bugs in the new islands system

* chore: remove redundant active_set_offset field

examples2d/one_way_platforms2.rs

@@ -105,7 +105,7 @@ pub fn init_world(testbed: &mut Testbed) {
         }
 
         for handle in physics.islands.active_bodies() {
-            let body = &mut physics.bodies[*handle];
+            let body = &mut physics.bodies[handle];
             if body.position().translation.y > 1.0 {
                 body.set_gravity_scale(1.0, false);
             } else if body.position().translation.y < -1.0 {