Finish writing face construction routine by using rpn as key
Write and test a rabbit_face_vertices function

Faces are keyed by their rational number position (rnp), three integers
labeling the coordinates of the face's center. The depth and orientation
of the face are inferred from the least significant active bit in the
rnp. Then the rnp of vertices can be calculated. The function returns
them in counter-clockwise order.
Rename vertices function to geom, and return depth and axis as well
Write a face ordering that puts subfaces next to their parents