Resolve "Make `whnf()` less agressive" ✨️

Closes #34 🐔️👍️
Approved-by: aalbert <augustin.albert@bleu-azure.fr>
Approved-by: v-lafeychine <vincent.lafeychine@proton.me>
Approved-by: loutr <loutr@crans.org>

🦀️🍰🦀️🍰🦀️🍰

* feat: coverage

* chore : refactor

* chore : better comments

* chore :  more comments

* feat : more improvements

* feat: uwu

* Feat : don't traverse whnfs when they're syntactically equal

* feat : make whnf() less agressive