Add week-of-year to semantic skeletons

[sffc]

Week-of-year should be added to semantic skeletons, both in CLDR and in ICU4X.

Previous issue: #488

Week-of-year requires plurals support. Plurals are mostly supported in the data model already, so this should not require a breaking change.

The following fixture tests were going to be deleted in #5640, and they should be covered by this issue:

    {
        "description": "Date time example that includes a week-of for a locale with plural variants as well as one with non default WeekData",
        "input": {
            "locale": "en",
            "value": "2016-04-17T08:25:07.000+05:00",
            "options": {
                "components": {
                    "year": "numeric",
                    "week": "numeric-week-of-year",
                    "hour": "two-digit",
                    "minute": "two-digit",
                    "second": "two-digit",
                    "time_zone_name": "offset"
                }
            }
        },
        "output": {
            "values": {
                "en": "week 17 of 2016, 08:25:07 GMT+05:00",
                "en-AU": "week 16 of 2016, 08:25:07 GMT+05:00",
                "en-GB": "week 15 of 2016, 08:25:07 GMT+05:00",
                "fil": "linggo 17 ng 2016, 08:25:07 GMT+05:00",
                "en-ZA": "week 17 of 2016, 08:25:07 GMT+05:00"
            }
        }
    }

[sffc]

Previously, week-of-year was handled internally in the datetime formatter. However, in the semantic skeleton world and with flexible input types, I would rather move the week number calculations out into the input type. For example, I would rather have something such as

pub struct WeekDate<C: WeekCalendar> {
    inner: C::WeekDateInner
}

struct ArithmeticWeek {
    pub(crate) year: i32,
    pub(crate) week: u8,
    pub(crate) weekday: Weekday, // 1-7
}

pub struct GregorianWeekDateInner(pub(crate) ArithmeticWeek);

impl WeekCalendar for Gregorian {
    type WeekDateInner = GregorianWeekDateInner;
}

The functions for converting between Date and WeekDate involve data and locale preferences (i.e. WeekCalculator).

This has a number of advantages, including removing the dependence on the week calculator data from datetime (#4340).

(this is not a 2.0-issue, but CC @Manishearth @robertbastian @zbraniecki)