Wumpus

Copperbox revision 666.

I've changed the name of Core.VSExtra to Core.Geometric. While the new name is pretty arbitrary, I wanted a place in the module hierarchy to add (dependency-free) type classes - the old VSExtra suited this.

I've added a new class Congruent - unfortunately the current instance for line segments is bringing in OverlappingInstances and UndecidableInstances as I can't work out how to get the type class constraints smaller than the instance head.