REF: use asi8 instead of _data in PeriodArray
@TomAugspurger @jorisvandenbossche discussed on the call a couple weeks ago.  Reference `asi8` instead of `_data` or `_ndarray_values` is the most-explicit option available.

I would also be in favor of getting rid of `_data` directly and setting `asi8` in `__init__`, but holding off on that pending consensus.

Also re-implemented some simplifications that got lost in rebasing during the PeriodArray marathon.