Refactor to make the code more idiomatic

[hanako-eo]

This is a refactor to make the code more idiomatic (even if the code works in the previous version, this refactored version is intended to make the rust code "clearer" and closer to code written by someone used to the language).

The aim of this pr is not to change the code enormously or add new features, but more to make idiomatic changes. All major changes (such as the file_path which changes from String to Rc<Path>) will be justified or justifiable (if justification has been forgotten). Some comments will be added to give tips for more understandable (and idiomatic ^^', I'm thinking of let mut module = Module::default(); in the parser.rs file) code, but of course this is nothing mandatory.

(the refactor isn't finished yet)