Resolve "Add axioms" ✨️

Closes #70 🐔️👍️
Approved-by: v-lafeychine <vincent.lafeychine@proton.me>
Approved-by: loutr <loutr@crans.org>
Approved-by: aalbert <augustin.albert@bleu-azure.fr>

🦀️🍰🦀️🍰🦀️🍰

* fix: change lifetime in functions for axiom-term construction

* docs: refer to payloads in builder documentation when missing item documentation

* Apply 1 suggestion(s) to 1 file(s)

* Apply 1 suggestion(s) to 1 file(s)

* chore(kernel): remove unused pointer

* Apply 1 suggestion(s) to 1 file(s)

* Apply 1 suggestion(s) to 1 file(s)

* Apply 1 suggestion(s) to 1 file(s)

* Apply 1 suggestion(s) to 1 file(s)

* Apply 1 suggestion(s) to 1 file(s)

* chore(kernel): remove unused file

* chore(kernel): coverage

* chore(kernel):add coverage

* fix(kernel): fix comment

* Apply 1 suggestion(s) to 1 file(s)

* chore(kernel): move axiom.rs back to root

* feat(kernel): have working axioms

* feat(kernel): better Axiom type

* fix(kernel): fix memoization issue with WHNF and reductions

* chore(proost): make annoying clippy happy

* chore(parser): make annoying clippy happy

* chore(kernel): make annoying clippy happy

* fix(kernel): fix type inference memoization issue with declarations

* fix(kernel): seamless builder interface for variable/declarations

* fix(kernel): remove unused function

* feat(kernel): generalize recursors reductors

* fix(parser): parsing of vars

* fix(kernel): recursor reduction

* fix(parser):tests

* Revert "feat(all): remove `Var` builder"

This reverts commit 9380961f.

* feat(kernel): add recursor reduction

* feat(kernel): add axiom declarations

* chore(parser): remove outdated TODO

* chore(kernel): add TODO

* feat(kernel): add type-checking for axioms

* feat(kernel): add axiom constructor

* feat(all): remove `Var` builder

* fix(kernel): add axiom constructor