Use named fields for `{ast,hir}::ItemKind::Impl`
Currently, the widely used `ItemKind::Impl` variant is a tuple with seven fields. I want to add an eighth in #68140, which means I have to update most matches on this variant anyways. Giving a name to each field improves readability and makes future changes of this nature much simpler.
This change will cause several tools to break. I will fix them once this is merged.