Consolidate redundant references to GlobalCurve

[hannobraun]

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

Currently, there are references to GlobalCurve in two different places: Curve and GlobalEdge. Both of those objects are referenced by HalfEdge. An instance of HalfEdge is only valid, if the GlobalCurve being referenced is the same one in both places. This is verified by a validation check.

This is another part of the object graph that is redundant and can be simplified. I have the following in mind:

1. Remove the reference to GlobalCurve from Curve. (Remove reference to GlobalCurve from Curve #1607)
   This reference is currently used in the approximation code, to ensure that coincident Curves result in the same approximation. It shouldn't be too much of a problem to adjust this, maybe by moving that approximation caching logic up to the HalfEdge level.
2. Remove GlobalCurve. (Merge GlobalCurve into GlobalEdge #1610)
   Once the previous step is done, the only remaining purpose of GlobalCurve (unless I'm forgetting something) is to track its identity, so we can uniquely identify edges that are supposed to be the same. This role is redundant, since GlobalEdge already provides that. At this point, it should be possible to remove GlobalCurve.
3. Merge Curve into HalfEdge. (Merge Curve into HalfEdge #1616)
   At this point, Curve only contains a SurfacePath and nothing more, meaning its further existence as a separate object serves no purpose. It can just be inlined into HalfEdge.
4. Rename SurfacePath to Curve. (Rename SurfacePath to Curve #1617)
   The previous item frees up that name, so it can be taken over by the more awkwardly named SurfacePath.

I'm going to start working on this shortly, as part of my larger simplification effort (#1589).

[hannobraun]

I've recognized some more opportunities while I started working on this. Added two more items to the list in the issue description.