Fixes assert_eq always passing for U256

[esdrubal]

Description

The issue was occurring because get_items_for_type_and_trait_name was comparing an absolute call path to an external relative call path, when the second should also be absolute.

compile_fn now checks if method is trait dummy.

These changes should avoid the silent failures where a dummy method is used,
instead of the proper implementation.

Closes #4989

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[anton-trunov]

It seems that one more way to fix this for a lot of cases without introducing the is_trait_method_dummy flag would be to check if the function body is empty and the return type is not (). Although, there is one more case to handle, legitimate functions like this one:

fn foo(...) {
}

On the other hand those functions are kind of useless and we might as well prohibit functions with the empty body and require the user to write something like this

fn foo(...) {
    return ()
}

Just some food for thought :)