Port upstream code

[Lusito]

I've written a simple tool named updiff, which is located in the packages/updiff folder to help with comparing the current state of this project against upstream code of Erin Catto.

I've already ported a lot of the changes in recent upstream version, but during this effort, I improved the tool a lot and it seems I have missed some places.

This is quite stupid work, but it needs to be done: Compare code and port everything that has not been ported yet.

Here's the list of all modules to check:

• [x] b2_body
• [x] b2_broad_phase
• [x] b2_chain_circle_contact
• [x] b2_chain_polygon_contact
• [x] b2_chain_shape
• [x] b2_circle_contact
• [x] b2_circle_shape
• [x] b2_collide_circle
• [x] b2_collide_edge
• [x] b2_collide_polygon
• [x] b2_collision
• [x] b2_common
• [x] b2_contact
• [x] b2_contact_manager
• [x] b2_contact_solver
• [x] b2_distance
• [x] b2_distance_joint
• [x] b2_draw
• [x] b2_dynamic_tree
• [x] b2_edge_circle_contact
• [x] b2_edge_polygon_contact
• [x] b2_edge_shape
• [x] b2_fixture
• [x] b2_friction_joint
• [x] b2_gear_joint
• [x] b2_island
• [x] b2_joint
• [x] b2_math
• [x] b2_motor_joint
• [x] b2_mouse_joint
• [x] b2_polygon_circle_contact
• [x] b2_polygon_contact
• [x] b2_polygon_shape
• [x] b2_prismatic_joint
• [x] b2_pulley_joint
• [x] b2_revolute_joint
• [x] b2_rope
• [x] b2_settings
• [x] b2_shape
• [x] b2_timer
• [x] b2_time_of_impact
• [x] b2_time_step
• [x] b2_types
• [x] b2_weld_joint
• [x] b2_wheel_joint
• [x] b2_world
• [x] b2_world_callbacks

I will update the checkboxes when they've been "upstreamed".

[Lusito]

We're up to date with this commit: https://github.com/erincatto/box2d/commit/95f74a46c077825478f989df6a81003ca538c2c2

Testbed could also be updated, but that has less importance right now.