Performance for TimedeltaArray.__iter__ and PeriodArray.__iter__

[TomAugspurger]

Right now DatetimeArray.__iter__ does it's boxing in chunks. We could do something similar for TimedeltaArray and possibly PeriodArray.

[jbrockmendel]

There is a tslibs.timedeltas.ints_to_pytimedelta(arr.asi8, box=True) that will make this easy to do for Timedelta; nothing analogous for Period