Fix sleeping and collision event issues

[Jondolf]

Objective

Fixes #161.

There are currently several issues with how collision events, CollidingEntities and sleeping are handled.

• Removing a collider from underneath a sleeping body will leave that body floating in the air, because from the engine's perspective, the bodies aren't penetrating.
• When a body is marked as sleeping, all of its collisions are discarded, and the CollisionEnded event is sent despite the bodies still being in contact from the user's perspective.
• Collision events are being sent only if the bodies are penetrating during the last substep during a physics frame, which is often not the case, because most collisions should be solved by then. This can lead to missed collision events and issues with CollidingEntities not being updated.

Solution

This PR aims to fix all of these issues listed above.

Collision events are handled in a much more robust way. Contacts has during_current_frame, during_current_substep and during_previous_frame properties that are used to track when to send each type of collision event or update CollidingEntities. This also lets us remove PreviousCollisions.

Collisions against sleeping bodies are also handled better. When a collision against a sleeping body ends (through movement or despawn), the bodies are woken up. Collisions between bodies are also not discarded when they start sleeping.

[emarcotte]

BTW I updated my little game jam game to main after this merge and it worked very well. Thanks!