9-add-a-basic-type-checker ✨️

Closes #9 🐔️👍️

🦀️🍰🦀️🍰🦀️🍰

* Fix : clippy

* Chore : document `TypeCheckingError`

* Chore : remove useless mut

* Feat : make `Context` private

* Feat : simplify `check`

* Chore : Remove old comment

* Chore : remove eta-expansion for functions to please the big guy

* Chore : comment error

* Chore : remove `println`

* Chore : add doc for `TypeCheckingError`

* Fix : put derive before comments

* Feat : add more coverage

* Fix : golf `whnf()`

* Fix : `polymorphism` test

* Fix : `pub_crate` for  `substitute`

* Chore : CamelCase

* Feat : add `Type_checking_error`

* Chore : move `normal_form` and `whnf` to `term.rs`

* Chore : rename tests

* Chore : remove debug `println`

* Chore : Add documentation

* Feat : use `whnf()` instead of `normal_form()` for conversion

* Chore : resolve ill-typed `complex_subst` test

* Feat : remove `Val`, correctly substitute terms.

* Chore : remove `classic_term.rs`

* chore(Context): Rename

* chore(type_checker): Apply clippy suggestions on tests

* fix: Remove Term's Abs/Prop names

* chore: format + derive Default for Ctx

* chore: Reorganise type_checker + derive Default for Ctx

* Feat : add documentation for `type_checker.rs`

* Chore: merge `impl Val`s

* Chore : remove debug prints in tests

* Chore : mark `Err`s with `//TODO #19`

* Chore : Move `Val` to the top of the file

* Fix :  remove `set_var` in tests

* Fix : `polymorphism` test

* chore: move everything to their respective `impl`

* fix: avoid `assert` in `is_def_eq`, move Val-related methods

* chore : make `conversion` a method

* Fix: make `normal_form` a method

* fix:  use `self` for `eval`

* Fix : Add issue reference

* Feat : remove `panic!`s, use `Result` monad instead

* Chore : rename `Ctx::empty()` to `Ctx::new()`

* Chore : remove duplicate `assert_def_eq`

* Chore : add `impl Term`

* Chore : remove `use Term::*`, add `use Val::*` instead

* Chore : remove `Into` trait for UniverseLevel

* remove useless `impl` for `UniverseLevel`

* Chore : remove prints

* fix : comment `Val`

* Fix : make `conv` into an `Into` trait

* fix : `Val` constructor names

* fix : imports

* chore : fix TODO

* fix : `lib.rs` imports

* fix : inference for lambda-abstractions

* chore : remove useless print

* chore : fix various comments

* fix : `is_universe`

* chore : add more comments to `conv`

* feat : simplify `quote`

* chore : add TODO

* fix : commentary for beta_reduction

* chore : comment Val

* chore : remove old comment

* fix : added `Ctx` type to store type of Vars during checking

* feat :  add polymorphic test which shows the need to add `Ctx`

* chore : please Clippy

* Fix : stop cloning environments for no reason

* fix : test

* chore : add comments

* feat : add `check` and `infer`

* fix: add string data to lambda-pi abstractions from classicTerm

* feat: add string data to pi/lambda-abstractions

* fix : made clippy happier

* chore : adapt existing code to rebase

* fix : conversion between Var and VVar

* fix : remove complex_subst from the def_eq test suite for now

* feat : added Ctx in preparation for type-checking

* this pleases the formatter

* Fix : conversion algorithm

* basic conversion algorithm (doesn't work)