refactor: Use metamodel and remove AST based traversal

[coder3101]

Includes Phase 3 of #130

It also brings in a lot of refactoring and removal of old AST based approach.

[AlexCannonball]

@coder3101 Hello, I'd like to explain, why it wasn't mod.rs in #145

https://doc.rust-lang.org/reference/items/modules.html#module-source-filenames

Prior to rustc 1.30, using mod.rs files was the way to load a module with nested children. It is encouraged to use the new naming convention as it is more consistent, and avoids having many files named mod.rs within a project.

I decided not to touch all the modules and focus on the Phase 1.

If you are aligned with the modern approach, we can plan refactoring to remove mod.rs files.

[coder3101]

I would actually prefer to keep using mod.rs. With this, all the files of a module are in a single directory which I prefer over the new convention. We also don't have any nested modules.