Optimize geometry conversion code

[TeodorVecerdi]

I tried to optimize the code at the expense of some readability, but it's still quite slow.
It might be worth looking into https://docs.unity3d.com/Manual/JobSystem.html and refactoring code to work with that. The biggest performance hit comes from a big nested loop that should be easy to parallelize.

Performance breakdown:

• GetDuplicateEdges: 66.2%
  • Self: 32%
  • List.get_Item: 15.8% (8,297,838 calls)
  • VectorUtilities.DistanceSqr - 12.8% (5,528,820 calls)
  • List.get_Count: 2.6% (1,385,660 calls)
• RemapDuplicates: 18.6%
• RemoveDuplicates: 3.3%
• RemoveInvalidDuplicates: 0.7%
• GetDuplicateVerticesMap: 0.3%