Switch to fourmolu for code formatting

[lehins]

In order to stay consistent with the main (at the moment the only) user of this package it needs to follow the same rules about the code style, because it will be Ledger's responsibility to maintain this package.

fourmolu configuration to be used can be found here: https://github.com/IntersectMBO/cardano-ledger/blob/master/fourmolu.yaml

CI action need to be added to enforce proper formatting with fourmolu