Static pure function interface

Added

• Added TestResult type which is returned from test() and can be checked directly
  for collisions or passed to intersect() to further calculate an intersection
• Added IntersectResult type which is returned from intersect() and can be used
  to calculate collision normals, separation distances, etc
• Added Headbutt.testAndIntersect() function which is a shortcut for testing and
  then calculating the intersection if a collision is found

Changed

• Changed Headbutt interface from interacting with an object with internal state
  to static pure functions (i.e. call Headbutt.test() instead of new Headbutt().test())