Support sweeping in arbitrary directions

[hannobraun]

At this point, sweeping is only possible along the z-axis. However, most of the code in the kernel that supports sweeping already works with arbitrary vectors. Sweeping in arbitrary directions might be possible to support without much work.

This would involve accepting a vector instead of a scalar in fj::Sweep, then passing that vector directly to the respective places in the kernel, instead of constructing such a vector from fj::Sweep's length field. Some changes might be necessary to internal kernel APIs. The places that need modifications should be fairly obvious after changing fj::Sweep, by following the trail of compiler errors.

This would need to be tested thoroughly, by adding new models that test different situations (sweep along positive z, sweep along negative z, sweep along a slanted vector, sweep sketches that were rotated with fj::Transform, etc.).

Labeling https://github.com/hannobraun/Fornjot/labels/good%20first%20issue, as at least the first steps of this work, as outlined above, should be fairly straight-forward and might serve as a good introduction to the fj library and the kernel. If this exposes any non-trivial problems, we can open follow-up issues for those.