Generalize Turtle module to create Paths as well as Diagrams

[byorgey]

Changes to the public API:

• The runTurtle function (and runTurtleT) now simply returns a raw TurtleState value
• drawTurtle does what runTurtle used to do, i.e. return a Diagram
• sketchTurtle throws away all the pen style information and simply returns a Path R2.

Internally I just added a few new functions for doing stuff with paths instead of diagrams, and did a bit of refactoring.

What do you think -- are these good changes to make to the API? Can you think of a more sensible/compact way to organize things? etc.?

[mgsloan]

Looks good to me!