[plt-scheme] faster testing for world programs

In 4.2.1, unit testing with image functions is quite slow. I'm  
guessing that this comes from pixel by pixel comparison of images.

It seems like an easy optimization would be for the images returned by  
the image functions to also carry a symbolic description, and for the  
equality testing to operate on the symbolic descriptions. Since the  
images are immutable it seems like that would work.

Or is there perhaps a better solution in the works? As it is, my  
students are commenting out test cases which isn't what we want.

Gregor