Remove many abbreviations

[giacomocavalieri]

This PR removes the use of various abbreviations:

• r#gen was renamed to this to follow an already existing convention in the codebase
• elems was renamed everywhere to elements
• when iterating over elements the loop variable is now consistently called element instead of elem or e
• there was one stray occurrence of typ that I renamed to type_ to follow the already existing convention
• retrn was renamed to return_ to be consistent with the choice of adding an underscore after names that would otherwise be Rust keywords
• t has been renamed to type_ when it references a type
• I've renamed a couple other single-letter lambda arguments to use more descriptive names
• I did quite a bit of renaming for the arguments of the functions is ast_folder, previously a lot of those were single letter names

[GearsDatapacks]

I think there is at least one place where we use retrn as an abbreviation for return. Maybe we should rename that to return_?

[giacomocavalieri]

Oh yeah great idea, I'll rename that as well

[lpil]

Thank you!