use something fast for memoization (dynprog, chapter 18)

[clayrat]

• dogfood with 2-3/RB/AVL-based map
• MMap: https://github.com/xavierleroy/coq-mmaps/
• FMap https://coq.inria.fr/library/Coq.FSets.FMaps.html
• https://github.com/arthuraa/extructures - still pretty slow

[clayrat]

https://gitlab.com/thewalker77/ssrmap

[clayrat]

Can be based on

• sketch up a finset/map lib #10
• Appel-Leroy #8