…AY is not in order

The recurrence iterator has a problem where it makes occurrences in the same order as given in
BYMONTHDAY or BYYEARDAY, which doesn't make sense - it should always return them in chronological
order.

This patch fixes it by pre-ordering the days that can be used when the month or year changes.
A day that is valid for one month/year may be different or not valid for the next month/year, so
the days need to be recomputed each time.