Hy-lists and Wumpus

Copperbox revision 940.

I've added some more functionality to hy-lists, mainly Foldable and Traversable instances for OneList.

The Picture type in wumpus-core has been changed again to make it a rose tree. Lists of child nodes are represented with OneList to prevent empty pictures being constructed. The change from binary tree to rose tree is so I can build multi-pictures - ones that share the same affine frame - more easily.