wumpus-basic

Copperbox revision 2168.

I've added support to convert the units for various drawing objects Image, PosImage, TraceDrawing etc.

The code is unfortunately more contrived than I had expected. Changing units in Wumpus-Core is just a functorial traversal, but this isn't the case in Wumpus-Basic. Also in Wumpus-Basic, the many of the objects are type synonyms with kind (* -> * -> *) so there is no opportunity to encapsulate conversion in a type class. Instead I've had to code the type specific cases with unique functions that look the same if you squint.

Wumpus-Rhythm has a compelling use for unit conversion, so it will be a test-case to gauge whether the new code is satisfactory if not pretty.