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/data/arena.rs

@@ -70,11 +70,8 @@ impl Index {
     ///
     /// Providing arbitrary values will lead to malformed indices and ultimately
     /// panics.
-    pub fn from_raw_parts(a: u32, b: u32) -> Index {
-        Index {
-            index: a,
-            generation: b,
-        }
+    pub fn from_raw_parts(index: u32, generation: u32) -> Index {
+        Index { index, generation }
     }
 
     /// Convert this `Index` into its raw parts.

src/data/coarena.rs

@@ -29,6 +29,10 @@ impl<T> Coarena<T> {
         self.data.get(index as usize).map(|(_, t)| t)
     }
 
+    pub(crate) fn get_gen(&self, index: u32) -> Option<u32> {
+        self.data.get(index as usize).map(|(gen, _)| *gen)
+    }
+
     /// Deletes an element for the coarena and returns its value.
     ///
     /// This method will reset the value to the given `removed_value`.