wumpus-drawing

Copperbox revision 2282.

Initial work on building relative paths.

Currently paths are built with commands that are interpreted rather than combinators. This might turn out to be simpler - the monad that interprets the commands is quite complicated, and it should be more memory efficient (whether or not this will matter much in practice). At the moment the code can just extend the path and decorate the path with insert patterned after the TikZ command of the same name.