Post-3.1 Enhancement: Optimizations for PhysicsEnginePlatformer

[pushfoo]

Enhancement request:

This should probably wait until after 3.0 or 3.1 since:

1. We need 3.0 out
2. It will involve breaking API changes

What should be added/changed?

• Consider caching collision checks for bool-returning functions
  • is_on_ladder()
  • can_jump()
• Collision group code might be worth encapsulating (ladders, walls, etc)
• Rename the collision group related methods

What would it help with?

• Clarity
• Efficiency