[racket] New turtle graphics package + a few questions

Thanks Sam and Matthias for the tips:  I've been able to clean up the
directory structure and
add a (save-movie) command that writes out an animated gif.

Example:
https://github.com/danprager/turtlegraphics/blob/master/docs/animated-circle.gif

Matthias wrote:
> Thanks, that's great. In what way did you find the existing one lacking?

I did start with that existing package, but initially wanted to simplify
the process of drawing pictures interactively, and boost the immediate
appeal for my kids.  Specifically, I wanted:
* input and output directly in Dr Racket's REPL window, rather than an an
"edit definitions-run-close graphics window" cycle.
* a realistic image of a turtle, rather than a triangle.

Based on observation of my kids I made improvements to enhance usability
and appeal.  Main additions:
* (undo) and (redo) to improve interactivity
* facility to change the turtle's image into e.g. a pink girl
* replay all the turtle's steps as an animation: (movie)
* (show-program)  to recap the interactive commands used so far for copying
into the definitions window

Of these (movie) is definitely the "wow" feature.

BTW: If someone would like to contribute, a bit-blt-based "color-in"
operation, that would be great.

I should add that after taking Shriram's recent online PLAI course I
restructured the whole thing around a sugared and desugared language.  This
approach allowed me -- amongst other simplifications -- to cleanly separate
out the chopping-out-into-frames needed for animation from the rest of the
code.  Previously both modes of movement were smooshed together.

Dan
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