Add ocamlformat and update CI pipeline

[shonfeder]

• Add an .ocamlformat config
• Reformat code base
• CI pipeline updates
  • Use github actions v2
  • Run formatting check
  • Add missing compiler versions
  • Enable dune-cache

I'll wait to merge this until #232 (and other substantial outstanding PRs?) are merged in.

In the meantime, @sonologico, whenever you have a chance, let me know if you are OK with these formatting, or if you have strong feelings by a different config.