impl `DoubleEndedIterator` for `ScheduleIterator` is incorrect

[Ten0]

It is sometimes useful to get the past expected runs of a cron expression.

So it would be great to have a Schedule::before, working the same as Schedule::after, but going backwards in time.

[Ten0]

Wait is that the DoubleEndedIterator impl for ScheduleIterator ?

In which case that is not a valid implementation of DoubleEndedIterator (which contractually means you can iterate both from the front and the back but it will yield the same elements), and these should instead be two different iterators.

[Ten0]

Actually I'm thinking using the Range types with ranges of datetimes would probably be the ideal interface :)

// all future events
schedule.events(start_datetime..)
// all past events, iterates in reverse
schedule.events(..end_datetime)
// all events between start_datetime and end_datetime excluded. Iterates from start to end, but implements DoubleEndedIterator
schedule.events(start_datetime..end_datetime)
// all events between start_datetime and end_datetime included (that is, if end_datetime is an instance of the schedule, it will appear). implements DoubleEndedIterator.
schedule.events(start_datetime..=end_datetime)

Note that schedule.events(start_datetime..) would not be equivalent to the current schedule.after(start_datetime..) in that it would yield start_datetime if it was an instance of the schedule.
We could have another method to exclude that (e.g. schedule.events_start_excluded(start_datetime..)).