stl writer, removing all 64/32s, renaming types

The following I did: 

* Renaming types: 
    - Mesh -> IndexedFaceSet
    - Vertex is a typealias of Vector3d

* New mesh types: 
    - Face
    - FaceSet

* Removing all 64/32s: 
    I'm not sure, whether you need them or not.

* Rudimentary diff between meshes