Validate that regions within a sketch don't intersect

[hannobraun]

Current Situation

A Region is a single continuous 2D region. For example, it might be a circle, or a polygon, or a polygon with holes. Sketch is the top-level 2D object in Fornjot, a 2-dimensional shape. It consists of multiple regions.

If the regions in a sketch were to overlap, that would make the sketch invalid. But this is neither documented nor enforced right now. We need a validation check for this.

Implementation

The new validation check would go into validate::sketch. There is already an existing check there, and numerous others in the sibling modules. Those can be used as a template for implementing the new one.

Related Issues

There are a few related issues that are currently open:

• Separate geometry from topology #2116 and Consider universal intermediate representation for geometry #2118 are about a fundamental redesign to how geometry is handled. If those were implemented first, this would simplify the implementation of this issue.
• Consider removing region interiors #2132 is about simplifying Region itself. If that were implemented first, it would also simplify the implementation of this issue.
• Improve validation infrastructure #2157 is about a refactoring of the validation infrastructure. If that were implemented first, it would change where the new validation check would go, and how specifically it would be implemented.

None of those issues are blockers for this one, but if work on any of them was ongoing by the time this issue is picked up, that work would have to be coordinated.