Wumpus

Copperbox revision 707.

I've added an elementary picture combinator `below` which does the obvious and draws one picture below another. More should follow e.g. above, and it does seem to validate the utility of the Picture type.

Also I've removed the code to explicitly manage the PostScript translation matrix. Current colour and other areas which the PostScript state is shadowed on the Haskell side need thinking about too.