Thin DateTime type in icu_timezone

[Manishearth]

#5533 (comment)

Conclusion: Option 4 (Thin DateTime and ZonedDateTime type in icu_timezone), potentially (separate discussion) reexport DateTime from icu_datetime

Thin DateTime type in icu_timezone

• icu_calendar becomes purely Date
• icu_timezone has time zones and times; possibly rename to icu_time
• IXDTF parsing can return a DateTime or ZonedDateTime, parser API lives in icu_timezone

[sffc]

@robertbastian pointed out that in FFI we need to do this

                .format_any_calendar(&icu_timezone::DateTime {
                    // Arc clone
                    date: date.0.clone(),
                    time: time.0,
                })

Possible solutions:

0. Keep the status quo
1. Make a private type in FFI that borrows things and implements GetField etc.
2. Option 1 but put it in icu_timezone
3. Change DateTime to be parameterized with AsRef fields

[Manishearth]

@sffc I think we have another option: adding Date::wrap_calendar_in_ref() that wraps the calendar in a Ref, returning a new borrowing date. The rest of Date's fields are a cheap clone and I wouldn't be opposed to guaranteeing that the DateInner types are all Copy. (or at least adding DateInner: Copy to wrap_calendar_in_ref()

[sffc]

Yes that is the correct solution. We solved the ownership problem in the Date type via AsCalendar.