Marching cubes index caching may be broken

[swiftcoder]

The linear hashed marching cubes algorithm is producing meshes with roughly the same number of triangles, but half the number of unique vertices compared to the basic marching cubes algorithm.

This likely means that my (overcomplicated) index cache isn't actually deduplicating all of the vertices that it should be. This issue is to verify whether the index cache is actually working correctly, and fix it if necessary. If it makes sense to simplify the index cache logic in the process, that'd be a general win.

[swiftcoder]

Closed by #10