Wumpus

Copperbox revision 715.

I've remade the Wumpus monad to be ReaderT over WriterT rather than StateT over WriterT.

While the original is a more accurate model of PostScript - changes to the graphics context are global (or a least delimited by gsave/grestore), the new monad allows me to control effects better. In Haskell changes are delimited by the Reader monad's local and are translated into PostScript within gsave/grestore blocks.

Also, I've gone through all the examples and made sure they compile and do something. Sometimes this has meant commenting out drawing operations that have become outdated but aren't yet replaced.