Dataset: now it officially supports setting a blessed object as an option. Some explicit stringifications are inserted to the code.
Script: now it officially supports setting a blessed object.
t/dataset, t/script: get_option() returning a blessed object is now tested both in list and scalar contexts.
Dataset: deprecate the experimental "set_join" feature.
PartiallyKeyedList: rename get_all() -> get_all_values() because we are likely to add get_all_keys() in the future.
PartiallyKeyedList: official support for get_at() in scalar context. No implementation change.