Rename `option_union_Some` → `union_Some`
See discussion at !432 (comment 87085)
This removes option_ if there's already something else in the name to disambiguate, here that is _Some.
We could also prefix everything with option_, but that would require more changes. For better or worse, I think this MR matches the consensus we also have for lists, maps, sets (e.g., having lookup_app_Some instead of list_lookup_app_Some).