refactor(data/set/countable): rename some lemmas (#15527)
* `set.countable_iff_exists_surjective` -> `set.countable_iff_exists_subset_range`;
* `set.countable_iff_exists_surjective_to_subtype` -> `set.countable_iff_exists_surjective`;
* `set.countable.exists_surjective` -> `set.countable.exists_eq_range`.
