Add in new 3D mesh SDF implementation

PTAL for https://github.com/soypat/sdf/issues/7 I used a slightly older version of the code I was talking about before, so it looks a little different. I'm also not entirely sure how to test against a dev version of a Go package without dealing with a lot of Go module stuff, so I'll take any advice you guys have on that. It compiles at least! It currently generates normals for vertices, faces, and edges, but effectively only uses the face normals right now. It'd involve tagging the distance measurements with their type and tracking that through all the distance comparison logic in case anyone's up for dealing with that