minor whitespace cleanup
export new getTurtlePath function
generalize runTurtle and add drawTurtle and sketchTurtle

The old runTurtle(T) resulted in a Diagram.  Now runTurtle(T) gives
you the raw TurtleState, and drawTurtle gives you a Diagram (what
runTurtle used to do) and sketchTurtle gives you a Path (ignoring the
pen style commands).

Closes #5.