Use static typing to distinguish between degrees and radians

[hannobraun]

Angles in the fj library are just represented as floating-point numbers in radians. This is error-prone, as users can assume those are degrees.

I think there should a an Angle type that uses radians internally, but provides constructors (from_deg, from_rad) and accessors (deg, rad) to make working with angles convenient and obvious. This type should also implement the common math operators.

Such a type will probably be useful in the CAD kernel too, so #122 is relevant.

[hannobraun]

This is a rather self-contained change that could be a good introduction to the fj crate, so I'm labeling it as https://github.com/hannobraun/Fornjot/labels/good%20first%20issue.

Here's what would need to happen to address this, basically:

1. Create an Angle struct, basically a wrapper around f64, and add it to the fj crate. Not sure where the best place would be, but just adding it to the root module, or a new angle module would be perfectly acceptable. We can always adjust later.
2. Add the aforementioned from_deg/from_rad/deg/rad methods. Implementing operators like std::ops::Add/Sub/... would be nice, but can be left for later.
3. Review the whole API of fj, and use the new Angle type everywhere to represent angles.

Ideally, we'd add an Angle type to fj-math, and just re-export that from fj. But as long as we haven't figured out #122, that's not an option. And in the meantime, doing what I described here is certainly better than the current situation.

[hannobraun]

Addressed in #619.