Objective

A lot of Avian's current contact types store some redundant data, aren't properly optimized, and aren't explicit enough about what they represent.

This PR is a clean-up pass to make the types a bit smaller and clearer.

Solution

Contacts

• Replace total_normal_impulse property with total_normal_impulse, total_normal_impulse_magnitude, and max_normal_impulse helpers
• Deprecate total_normal_force helper
  • You can just divide by the substep timestep
• Remove total_tangent_impulse property and total_friction_force helper
  • These could previously technically be wrong/misleading, since each contact point can have a different tangent direction, especially in 3D, and they can also change between substeps. The tangent impulse magnitudes of each individual point can still be accessed.

ContactManifold

• Rename ContactManifold::contacts to ContactManifold::points
• Use an ArrayVec with a capacity of 2 instead of a Vec to store 2D ContactManifold points directly on the stack
• Replace the local normal1 and normal2 with a single world-space normal
  • Internals only need the world-space normal, and it makes e.g. contact modification more straightforward
• Add total_normal_impulse and max_normal_impulse helpers

ContactData

• Rename ContactData to ContactPoint (represents a point in a contact manifold)
  • Another option would be ManifoldPoint like in Box2D; not sure which one is better
• Rename point1 and point2 to local_point1 and local_point2 for explicitness
• Remove normal1 and normal2 from ContactPoint, since the normal is already stored in the ContactManifold

Other

• Many documentation improvements
• Contact force debug rendering uses per-point forces instead of the total force for the contact pair

Migration Guide

There have been several changes to Avian's contact types to make them more optimized and clearer.

Contacts

• The total_normal_impulse property has been replaced with a total_normal_impulse helper method.
• The total_normal_force helper has been deprecated. Instead, just divide the impulse by the substep timestep.
• The total_tangent_impulse property and total_friction_force helper have been removed for being inaccurate/misleading. The tangent impulse magnitudes of each individual point can still be accessed.

ContactManifold

• ContactManifold::contacts has been renamed to ContactManifold::points.
• The local normal1 and normal2 have been replaced with a single world-space normal, pointing from the first shape to the second.

ContactData

• ContactData has been renamed to ContactPoint, since it specifically represents a point in a contact manifold, not general contact data.
• point1 and point2 have been renamed to local_point1 and local_point2 for explicitness
• normal1 and normal2 have been removed, since the normal is already stored in the ContactManifold.