Summer cleaning

[Raveline]

While I'm working on this repo, I might as well do this.

• Adds fourmolu to the CI;
• Use the foumolu configuration used elsewhere in the company;
• Apply it to the codebase;
• Drop the "Setup.hs" file;

TODO:

• Might be a good idea to discontinue support for older GHC version ?
• Might be a good idea to merge all the open PRs, rebase & update this one before merging it.

[arybczak]

Might be a good idea to discontinue support for older GHC version ?

Yes please. Feel free to add 9.8 and 9.10 to the CI, drop 8.8. While we're at it, you can also enable ImportQualifiedPost and add -Werror=prepositive-qualified-module to ghc-options.

Let's merge this after all outstanding PRs are merged.