Coptic and Ethiopic date conversions from iso fail for dates sufficiently in the past

[Manishearth]

    #[test]
    fn test_coptic_regression() {
        let iso_date = Date::new_iso_date(-100, 3, 3).unwrap();
        let coptic = iso_date.to_calendar(Coptic);
    }

This fails with:

---- coptic::tests::test_coptic_regression stdout ----
thread 'coptic::tests::test_coptic_regression' panicked at 'called `Result::unwrap()` on an `Err` value: TryFromIntError(())', components/calendar/src/coptic.rs:196:37
stack backtrace:
   0: rust_begin_unwind
             at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/core/src/panicking.rs:143:14
   2: core::result::unwrap_failed
             at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/core/src/result.rs:1785:5
   3: core::result::Result<T,E>::unwrap
             at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/core/src/result.rs:1078:23
   4: icu_calendar::coptic::Coptic::fixed_from_coptic_integers
             at ./src/coptic.rs:196:20
   5: icu_calendar::coptic::Coptic::coptic_from_fixed
             at ./src/coptic.rs:207:30
   6: <icu_calendar::coptic::Coptic as icu_calendar::calendar::Calendar>::date_from_iso
             at ./src/coptic.rs:104:9
   7: icu_calendar::date::Date<A>::new_from_iso
             at ./src/date.rs:112:21
   8: icu_calendar::date::Date<A>::to_calendar
             at ./src/date.rs:125:9
   9: icu_calendar::coptic::tests::test_coptic_regression
             at ./src/coptic.rs:312:22
  10: icu_calendar::coptic::tests::test_coptic_regression::{{closure}}
             at ./src/coptic.rs:310:5
  11: core::ops::function::FnOnce::call_once
             at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/core/src/ops/function.rs:227:5
  12: core::ops::function::FnOnce::call_once
             at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

What's happening is that month in coptic_from_fixed is ending up negative.

The code we have does seem to match the code from the book, I suspect what's happening is that dates before the Julian epoch are messing up the math somehow. I haven't been able to figure out what's wrong though.

Worth investigating.

[Manishearth]

cc @pandusonu2 would you have time to look into this?

[sffc]

@Manishearth Can you assign this issue to a milestone, such as 1.x Untriaged?

[Manishearth]

Potentially fixable with https://github.com/unicode-org/icu4x/pull/2704