Faster manifold check

[Floppy]

By using assimp loader and models, we can make the manifold check faster. This PR at present gives at least a 5x speed improvement; loading is super fast, though the manifold check is still slowish as it's done in Ruby. Still a big increase.

Unfortunately due to a bug in assimp's vertex join postprocessing (see https://github.com/assimp/assimp/pull/6278), it currently fails, as all meshes are coming up as non-manifold because of repeated vertices. We'll need to wait for the assimp 6.0.3 release before we can merge this.