Deprecate Date::extended_year

[robertbastian]

We should make people get the extended year through YearInfo, so that they are aware of the other year values that are not the extended year. This is also currently the only method from YearInfo that's also exposed on Date.

[sffc]

I'm tiring of deprecated APIs in icu_calendar but won't block this if others agree.

[Manishearth]

I don't find this to be particularly necessary, since the name of the API is already somewhat clear that this isn't a regular year. Maybe my intuition on user behavior is incorrect?

I'm not strongly opposed.

My litmus test for deprecations like this is: will users be happy or annoyed to get a deprecation warning? Overall I think they would be annoyed here: it feels like an API consistency issue rather than a footgun issue.

That's not my only criterion, but it's a quick litmus test.

@hsivonen or @zbraniecki , thoughts?

[robertbastian]

I think they will be happy. If you're naively looking for a numeric year value on the API, you find extended_year and year. year returns something complicated, so you just use extended_year, which might not be what you want. Forcing users to look at the different values we provider in year is beneficial for the majority of users.