This needs a title

Highest priority:

• Continue fixing to_mesh_iter, which still doesn't yet handle branching because (for one thing) it never tracks depth properly in order to backtrack.
• Clean up twist - maybe make a struct or trait.
• Do transforms compose in the reverse of automata_scratch? This appears to be the case.

Important but less critical:

• Do I actually need EmptyRule or can I get rid of a bunch of extraneous nesting? RuleEval with empty meshes and no children is equivalent. I have used EmptyRule exactly zero times.
• Why must I repeat myself so much in these definitions?
• The notation for transforms is really cumbersome. Some syntactic sugar might go far.
• What patterns can I factor out? I do some things regularly, like: the clockwise boundaries, the zigzag connections, the iterating over a Vec<Vertex> to transform each element and make another vector.
• Docs on modules
• Grep for all TODOs in code, really.
• Look at everything in README.md in automata_scratch.
• Implement some of the tougher examples from the above too, e.g. the triple nested spiral. See examples.py.

If I'm bored:

• Fix links in tri_mesh docs that use relative paths & do a PR?
• Look in https://www.nalgebra.org/quick_reference/# for "pour obtain". Can I fix this somehow? Looks like a French-ism that made its way in.