Resolve "Unhandled integers" ✨️

Closes #41 🐔️👍️
Approved-by: belazy <aarthuur01@gmail.com>
Approved-by: aalbert <augustin.albert@bleu-azure.fr>
Approved-by: loutr <loutr@crans.org>

🦀️🍰🦀️🍰🦀️🍰

* chore(builder): Open issue (#94)

* fix(elaboration): Rebase issues

* chore(parser): Check every branch on unhandled parse_level

* perf(parser): Use try_fold instead of collect

* chore(parser): Remove useless wrapping on parse_level

* feat(parser): add coverage

* feat(parser): remove sum in the parsing of plus

* chore(kernel): add tests for  function

* chore(kernel): Add documentation for the  function

* fix(parser): Handle FromStr failure on usize

* chore(parser): Modify Kind::CannotParse to Kind::UnexpectedToken

* fix(kernel): fixed issues enlightened during the review

* Change the location of `max_depth` which is now a impl of Builder
 * Remove the corresponding tests, which will be added in the future
* The test for the depth is now made before the creation of the `Level` struct

* feat(level): Add max_depth in Builder