Generalize Turtle module to create Paths as well as Diagrams

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.?