Shapes that include circles can't be rotated

[hannobraun]

I've replaced the last line of the spacer model with this:

use fj::prelude::*;
spacer
    .rotate([1., 0., 0.], std::f64::consts::FRAC_PI_2)
    .into()

The result is this error message:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: ()', src/kernel/topology/faces.rs:123:64

It comes from this piece of code:
https://github.com/hannobraun/Fornjot/blob/baa977d71031d305f73dd730052e53b6f4ad0c6f/src/kernel/topology/faces.rs#L120-L124

This makes sense, as the definition of Circle can't support rotations in 3D space:
https://github.com/hannobraun/Fornjot/blob/baa977d71031d305f73dd730052e53b6f4ad0c6f/src/kernel/geometry/curves/circle.rs#L10-L20

I'm not sure what the best definition would be. Making radius a Point<3> again and adding a normal vector should do it, but would also be slightly redundant (radius and the normal vector would always have to be orthogonal).

Addressing this issue would involve deciding on a circle representation that is suitable for 3D, changing Circle, and updating all its methods accordingly. I don't think any other changes would be required besides that, but I might be wrong.