Calendar arithmetic support

[Manishearth]

For 1.0 we disabled the calendar arithmetic APIs because they had a lot of missing things and needed polish.

The full plan is to follow Temporal (specifically see PlainDate.add(), .subtract(), and .until(), as well as the actual spec docs). Temporal has put a lot of work into picking exact semantics here and we should rely on that work.

The tricky thing will be making it work for non-Gregorian calendars as well since the number of months may change from year to year. The current internal CalendarArithmetic APIs have the ability to do this via months_for_every_year() and month_year() though we may also want a more efficient one that returns an array-like object. Doing this whilst still being efficient may be hard.

I'm happy to mentor an implementation.