Conform rescript code to ecosystem's preferred filename/directory name structure

[quinn-dougherty]

Description:

• https://dev.to/yawaramin/a-modular-ocaml-project-structure-1ikd
• https://rescript-lang.org/docs/manual/latest/project-structure

[OAGr]

@quinn-dougherty I don't think I like this issue much, as an issue, because I don't think we can attack it in one go. I imagine it's part of a longer conversation, basically.