Used enum instead of dyn trait

[TobiasJacob]

At first I tried using Rc<RefCell<Primitive>>, which made it impossible to downcast without losing the reference.

Then I realised, that ParametricCell defined like in the following is all we need. Basically, the trick is to put the Rc<RefCell<...>> INSIDE the enum, not the enum INSIDE the Rc<RefCell<...>>

pub enum ParametricCell {
    Point2(Rc<RefCell<point2::Point2>>),
    Line(Rc<RefCell<line::Line>>),
    Arc(Rc<RefCell<arc::Arc>>),
    Circle(Rc<RefCell<circle::Circle>>),
}

Now everything works just nicely, and we dont need any weird serilization logic anymore. Type information can be added or removed from an Parametric-like object without altering its memory location or needing to clone it.

[dzervas]

looks good! 🎉