Consolidate redundant references to Surface

[hannobraun]

(This issue is part of a larger cleanup effort. See #1589.)

As of this writing, Surface is referenced by both Curve and SurfaceVertex. Those references must be consistent: a HalfEdge is only valid, if the Curve and the SurfaceVertex instances it references all reference the same Surface. In turn, all HalfEdges in a Cycle must (transitively) reference the same Surface, and all Cycles in a Face must do so.

This redundancy makes building and modifying these objects unnecessarily hard. There also need to be validation checks on every level from HalfEdge upwards to verify consistency. I believe it would be better to reference Surface only once, in Face. This would simplify the object graph a lot.

It would also mean that Surface is farther away from where it is often needed (for example, to approximate a curve in 3D space, you need the surface that it is defined on). But I think this can be addressed by structuring the code that uses Surface differently, on a case-by-case basis. I think that overall, this will be a worthwhile trade-off, as any additional complexity in code using Surface would be largely restricted to that code, while the additional complexity in the object graph affects the whole kernel negatively.

This issue is closely related to other cleanup efforts, namely #1525, #1570, and #1586. It will probably not be possible to address each of those in isolation, as the very complexity these issues aim to address tends to hinder them. Most likely, the issues will be addressed together, by picking low-hanging fruit from each of them, until all of them have been addressed. This is something I am actively working on right now.

[hannobraun]

This seems to be going much easier than expected! I made it most of the way there today (see the pull requests referencing this issue, listed above this comment) and I'm not aware of any serious problems that would prevent me from wrapping this up next week.