[racket] new visual tool for student program understanding

Over the summer, we built a new tool for visualizing program
execution: think of it as a complement to the Stepper.  It is similar
in that it shows you the steps of program execution, what they
evaluate to, and thus how the result is obtained (though it focuses
only at the level of user-defined functions, ignoring the primitive
operations).  However, the Stepper presents a linear view of the
computation, whereas the Tracer presents a tree-shaped view.

It's easiest to understand this by looking at an example, which you
can do from the documentation.  This also gives you instructions on
how to run the Tracer.

http://planet.plt-scheme.org/package-source/tracer/tracer.plt/1/7/planet-docs/tracer/index.html

Here are some other example programs that you can try out to see the
Tracer at work:

https://github.com/retief/racket_tracer/tree/master/demos/good_demos

Tracer was built by two Brown undergrad students, Jeanette Miranda and
William Zimrin.  Please let us know if you have any feedback!

Shriram