Make contacts deterministic across Worlds

[cBournhonesque]

Objective

• Fixes https://github.com/Jondolf/avian/issues/406

We noticed an issue where the Collisions were using the order of Entity directly to sort the pair of entities in a contact. The issue is that for networking, there is no guarantee that the pair of Entities that are replicated between a Client and a Server world are in the same direction.

Solution

• A solution is to keep using the aabb.min.x to order the two entities.
• There are a couple concerns:
  • we want to make sure that when we match with previous entity pairs, this is not an issue.
  • it's not a problem for previous_contacts here: https://github.com/cBournhonesque/avian/blob/ac1fd0269e04f19f6892c62019cbb9ad9c19444e/src/collision/narrow_phase.rs#L549 because we check both directions
  • it is a problem for match_manifolds here: https://github.com/cBournhonesque/avian/blob/ac1fd0269e04f19f6892c62019cbb9ad9c19444e/src/collision/narrow_phase.rs#L571 which is why we modify the function
  • we want to make sure that this doesn't change how CollisionStarted and CollisionEnded events are sent, even if the entities change their order during a contact. we should probably add a test for this
  • it should be fined because those events are based on the Contacts struct that is generated during the narrow phase, and we made sure that contact matching still works independently of past entity order

Test

• tested in lightyear that the simulations are now completely deterministic
• probably need to test that contact matching works correctly, even if the aabb.min.x of the two entities gets swapped during the contact

[cBournhonesque]

Maybe the docs should be improved as well to make clear that:

• Collisions has the keys in entity order
• Contacts has the entities in aabb order (and it should probably use aabb.min.y as secondary sorting key?)