Refactor used attributes

[cristianoc]

No description provided.

[cknitt]

This looks great! Thanks!

But somehow it still doesn't do the trick.
If I enable "as" in Bs_ast_invariant.is_bs_attribute, I still get all the "unused attribute" warnings for @as in untagged variants, even though you added the Used_attributes.mark_used_attribute call there.

[cristianoc]

Probably the check for unused attributes runs before the untagged variants are processed.

[cristianoc]

Seems to be working with the last commit.

[cknitt]

Thanks a lot! 🎉

I pushed another commit because code formatting was incorrect, but it seems it is only checked together with the syntax tests (which are only run if something in the syntax package changed).

[cknitt]

Will also add a CHANGELOG entry, but need to rebase for that.