Refactor

I think a scene really wants to an ast-like thing that can be evaluated, given coordinates for its free points, into a concrete array of (coordinate) geometry objects, which can in turn be easily rendered. #2 is part of this, but not all of it. Plan:
- `euclid-scene`: the models & ast-building interface
- `euclid-draw`: evaluate a Euclidean geometry scene down to coordinate geometry pieces. The output here should allow for a mapping between the incoming nodes and the resulting items. Also, this needs a better name... and `euclid-descartes` is probably too cute for its own good.
- `euclid-render-svg`: render. use a vdom approach here.

Question:
- What's the simplest & cleanest way to handle styles/classes?