Flesh out 2D subsystem

[elalish]

We currently use Polygons as input to several Manifold constructors, and they could easily be output as well (think Slice), but we have no way to operate on them in 2D. I would like to think of Polygon as the 2D equivalent to Mesh, and I'd like to create a CrossSection class that is the 2D equivalent to Manifold.

A CrossSection will always be geometrically-valid (non-overlapping), as any input Polygons will be automatically fixed up according to winding number. Clipper lib seems like a good candidate to support useful 2D operations. I propose we adopt the "strictly positive" fill rule; this will allow Boolean-style operations simply by adding Polygons with the proper winding direction. This class will be totally independent of the Manifold class; they can simply interact through exported Polygons.