Wumpus

Copperbox revision 634.

I've now got a passable arrowhead coded in coordinate-free style. I've had no insights into how to add frames to the affine transformations, to get a workable rotation function I defined a rotateAbout function that is just the common TRT^-1 transformation and takes the rotation origin as a parameter.

Getting the arrowhead to work has reinforced a particular clumsiness in Wumpus where I can't translate all the points in a figure (polygon, etc.). Using fmap doesn't work because polygons are parametric on the numeric inside (Point a) rather than the point itself. Looks like I need a Pointwise typeclass.