Convert edge builder API into partial edge API

[hannobraun]

This is another step towards addressing the object identity problems that are currently holding up #1079.

[hannobraun]

This is failing the CI build. It's a validation error, because the half-edges of a cycle do not connect (caught by the export validator in the star model). There's only a minuscule difference between vertices that are supposed to be the same, so it's a floating-point accuracy issue. It seems to start with this commit: 0c7c508

I don't think that this commit actually introduces the bug. I might be mistaken, but I think it just exposes a bug that was already there. The vertices were already computed redundantly, and it just happened to work. The new code just happens to compute slightly different vertices redundantly, therefore exposing the bug.

This is exactly the kind of bug that would no longer be possible, if validation were based on object identity, not object equality. Then it would always cause a validation error, hence making it impossible for such a bug to hide. It's one of the reasons I want #1021, and why I'm doing all this work with the builder / partial object API.

I'll try to locate and fix the buggy piece of code, but I might not be able to finish that today. If so, I'll leave this open until next week.

[hannobraun]

I've located the bug and pushed a commit that fixes it before it is uncovered (meaning all the commits in this pull request should pass the full test suite). Here it is: 73bdcdb

The fixed code isn't exactly beautiful, but it's fine for now. It's in a builder I have yet to update, so I'll have an opportunity to clean it up soon.