Make an IxdtfParser type

[robertbastian]

Based on #5735

[sffc]

Praise: Thanks for keeping the function names the same so it is easy to review.

[robertbastian]

Hmm, how do I load all AnyCalendar data upfront?

[sffc]

Just put in the bounds like usual:

https://unicode-org.github.io/icu4x/rustdoc/icu/calendar/enum.AnyCalendar.html#method.try_new_unstable

We do this in the DateTimeFormatter constructors, too (they are at the end of the list):

https://unicode-org.github.io/icu4x/rustdoc/icu/datetime/struct.DateTimeFormatter.html#method.try_new_unstable

[sffc]

Oh, since it is cross-crate, you will also need the AnyCalendarLoader thing.

[sffc]

Oh, but you don't know the calendar until parsing time. Hmm.

Easy way out is to pass the AnyCalendar provider into the parse function, but that's annoying.

Maybe you can save a closure that calls the AnyCalendar constructor as a field of the parser? You'd need to put a lifetime on the parser object, I think.

[sffc]

Or you could eagerly load all the markers into payloads and then make your own internal DataProvider that reads those payloads when calling AnyCalendar::try_new_unstable. Hmm. Maybe we want that to be a type in the icu_calendar crate, since it seems kind-of silly that AnyCalendar::new might need to load the same payload over and over, and you can't save it between loads.

#5739

[sffc]

#5740