- `curve_horn_*`: actually *combine* geometry properly.

- Why is MeshBuilder so slow?  Am I certain I'm not giving it a lot of
  redundant geometry?

- 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
- Lots of Rust-kosher refactoring (once I understand Rust better)
- Grep for all TODOs in code, really
- Actual Rust-style docs!

- Fix links in tri_mesh docs that use relative paths & do a PR?