internal: Add `GenericParamList::to_generic_args` and `{TypeParam,ConstParam}::remove_default` APIs
Also fixes `generate_impl` not removing the default const param value, though it seems that no one has encountered or reported that issue yet 😅
This initially started out as refactoring `utils::generate_impl_text_inner` to understand it better (which was the reason for adding `{TypeParam,ConstParam}::remove_default`), but ended up also finding another place that needed `GenericParamList::to_generic_args`, hence its addition in here.
910c565b17