Resolve "Term definitions" ✨️

Closes #2 🐔️👍️
Approved-by: belazy <aarthuur01@gmail.com>
Approved-by: loutr <loutr@crans.org>
Approved-by: aalbert <augustin.albert@bleu-azure.fr>

🦀️🍰🦀️🍰🦀️🍰

* feat(term): Complete beta-reduction

* [WIP] feat(term): Improve beta-reduction + Add tests

* feat(term): Early beta-reduction

* feat(term): Add early Term struct + Display trait