Reimplement Diagrams.TwoD.Ellipse in terms of Diagrams.TwoD.Arc

[fryguybob]

(Imported from http://code.google.com/p/diagrams/issues/detail?id=53. Original issue from byor...@gmail.com on September 8, 2011, 08:26:47 PM UTC)

We are currently in the awkward situation that almost every shape function in the standard library returns any instance of PathLike, EXCEPT 'circle' and friends, which can only return Diagrams. This is because circle is implemented in terms of an Ellipse primitive rather than a path. However, now that we can generate approximate circular paths using the Arc module, there's no particular reason to keep doing this. The benefit of switching is that 'circle' and the other functions in Ellipse would now work just like all the other shape primitives, as users would expect. I don't see any downside (other than throwing away all the nice work in the Ellipse module, but that's life).

To clarify, the API of the Ellipse module should stay exactly the same, except that Diagram return types should be generalized to PathLike p => p.