Refactor core grammar to use recursion-schemes

[hhefesto]

Original Issue: sfultong/stand-in-language#8

I've created a branch called recursion-schemes, and it's super slow. If at least the type checker can't be made reasonably fast, this idea should probably be abandoned.

[sfultong]

see https://github.com/sfultong/stand-in-language/tree/recursion-schemes

[hhefesto]

Leaving this note for my future self (or anyone who wants to work on this):

• https://www.reddit.com/r/haskell/comments/ezt7ur/trying_to_understand_the_role_of_the_y_combinator/
• https://blog.sumtypeofway.com/posts/introduction-to-recursion-schemes.html
• https://www.schoolofhaskell.com/user/bartosz/understanding-algebras