Add Helper Functions to Create Areas Based on Shapes

[Chris3606]

Currently, a user can create an Area based on a shape by doing new Area(Shapes.GetCircle((1, 2), 3)). While this works, it isn't obvious that users would need to look in the Shapes class to create an area based on that shape, so some forwarding functions might be nice here.

One possible downside, though, is it might promote inefficient code. Consider if a user has a function which takes an IEnumerable<Point>, and does this: MyFunc(Area.FromCircle((1, 2), 3). This negates the benefits of the custom enumerator and lazy evaluation you get with Shapes.GetCircle() and causes quite a large amount of temporary allocation.