ensure uniform code style with an automated formatting tool

[m4lvin]

Candidates:

• https://github.com/tweag/ormolu
• https://github.com/lspitzner/brittany
• https://github.com/fourmolu/fourmolu
• https://github.com/haskell/stylish-haskell

plan:

1. choose a formatting tool
2. format everything in a separate commit
3. include and run the tool in CI

[foxyseta]

Would be happily to implement this before #24, as to make sure to work on a formatted codebase. How about (f)o(u)rmolu?

[m4lvin]

I have not made any decision on this, and there is other non-public code waiting to be merged some day before tackling this. So for now please ignore this / use whatever formatting works for you but try to avoid stylistic edits unrelated to your main changes.