Added positions.rkt, a simple representation of positions.
Add drawings.rkt, support for the basic drawing operations.
Add support for a few more color types.
Update the instructions to set the foreground color to match the
"color as integer" model that is currently the only one we support.
Add check-list and hsv?  (hsv? should be moved to colors.)
Minor cleanup.
Move check-list to utils.rkt.
Add 'all', 'left-section', and 'right-section'.
Add 'image-load', 'image-stroke-selection', 'image-show', and such.
Add foreach!
Include positions and drawings.
Move hsv? back to colors.rkt from utils.rkt.