Add support for non-ISO yearmonths and monthdays

[Manishearth]

This adds support for non-ISO yearmonths, as well as some non-ISO monthdays.

It currently does not support this for Ethiopic or Chinese/Dangi, since it needs working extended years.

Progress on #412, fixes #450

[Manishearth]

I used a little search script to generate the dates here:

use icu_calendar::Date;
use icu_calendar::Ref;
use icu_calendar::cal::HijriTabularEpoch;
use icu_calendar::cal::HijriTabularLeapYears;
use icu_calendar::types::MonthCode;
fn main() {
    let iso = Date::try_new_iso(1972, 12, 31).unwrap();
    let calendar = icu_calendar::cal::HijriSimulated::new_mecca();
    let calendar = Ref(&calendar);
    let iso_converted = iso.to_calendar(calendar);
    println!("{:?}", iso_converted);
    'outer: for i in 1..=12 {
        let month = MonthCode::new_normal(i).unwrap();
        let day = 30;
        let mut year = iso_converted.extended_year();
        if i >= iso_converted.month().month_number() {
            year -= 1;
        }
        while (iso_converted.extended_year() - year) < 1000 {
            let attempted_date = Date::try_new_from_codes(None, year, month, day, calendar);
            if let Ok(date) = attempted_date {
                println!("Found {:?} (ISO: {:?})", date, date.to_iso());
                continue 'outer;
            }
            year -= 1;
        }
        println!("No workable year found!");
    }
}

[Manishearth]

I think we should try and fix Chinese after unicode-org/icu4x#6800 or unicode-org/icu4x#6762 lands.

[nekevss]

In general, looks good to me. Have a couple doc specific misspellings / nits, but nothing blocking.

[nekevss]

nit: spelling