Wumpus

Copperbox revision 706.

I've added a Picture datatype for compositional images. The Picture type is derived from the one in Haven. I've rather overlooked Haven as a source of inspiration, largely because the geometrical stuff is on the foreign side of the FFI, but today I read then Haven chapter in Antony Courtney's thesis today and the Picture type seems very promising. I've been struggling to avoid composing images syntactically (i.e making a union algebraic datatype of the Line, Polygon etc. types). Haven's Picture type looks like a solution.