Add the ability to modify the contact points seen by the constraints solver

[sebcrozet]

This adds the ability to modify the contact points after they are generated by the narrow-phase but before they are seen by the constraints solver. This allows the simulation of various interesting effects like conveyor belts and one-way platforms:

This can also be used for fine-grained friction and restitution coefficient modification (e.g. to simulate multiple materials on a single collider).

The contact manifold now contains a user-defined piece of data that can be updated by the contact modifier. This is useful to add some updates that depend on the previous state of the simulation (as needed by one-way platform).

The two traits for filtering intersection and contact pairs have been removed in favor of a single PhysicsHooks trait that combines contact filtering, proximity filtering, and contact modification. This will also allow use to add more hooks in the future without requiring no arguments to be passed to the PhysicsPipeline::step method. This is a breaking change.

In order to tell the narrow-phase that contacts must be changed the contact manifold to be modified must be given the SolverFlags::MODIFY_SOLVER_CONTACTS flags. This can be achieved either using the contact pair filtering function, or by calling ColliderBuilder::modify_solver_contacts(true) on the relevant colliders. The latter option will be slightly more efficient.

Fix #24