Partially resolve "Pretty printing of terms" ✨️

Closes #79 🐔️👍️
Approved-by: aalbert <augustin.albert@bleu-azure.fr>
Approved-by: belazy <aarthuur01@gmail.com>
Approved-by: v-lafeychine <vincent.lafeychine@proton.me>

🦀️🍰🦀️🍰🦀️🍰



* test: provide tests for parenthesis omission and axiom printing

* feat: less parentheses in printing

* feat: use method `is_certainly_closed` in pretty printing

* chore(pretty): move pretty-printing functions into a separate module

* feat(kernel::memory::term): in pretty printing, associate variables to letters instead of numerical identifier

* fix: no space before ':'

* fix: parenthesis for application

* fix: prettyprint renamed in pretty_print + doc

* fix: cases for Prop display. doc: PrettyTerm and Term::prettyprint

* feat: add PrettyTerm structure for easy display manipulation

* fix(prettyprinting): less parenthesizing

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

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

* fix(prettyprint): comment as clippy wants & self by value as asked by clippy

* fix(prettyprint): variable names corrected

* feat(prettyprint): WIP on new prettyprint. WARNING: Variable names aren't always correct in the argtype.

* Resolve: "Add support for non-nix users" (79) ✨️
Closes #79 🐔️👍️
Approved-by: belazy <aarthuur01@gmail.com>

🦀️🍰🦀️🍰🦀️🍰

* fix: Specified which version is used (1.68.0 nightly 2022-12-27)

* feat: Add "nightly" in the rust toolchain