anchor: World:getCallbacks
  Returns functions for the callbacks during the world update.
  beginContact, endContact, preSolve, postSolve = World:getCallbacks()
  beginContact function Gets called when two fixtures begin to overlap.
  endContact function Gets called when two fixtures cease to overlap.
  preSolve function Gets called before a collision gets resolved.
  postSolve function Gets called after the collision has been resolved.