Add Date::new_from_iso(date_iso, X) example for each calendar X

[andrewpollack]

Adding a Date::new_from_iso(date_iso, X) example at the top level of each calendar.

• Skipped Japanese, leaving until future update when I understand examples for it better

---Below issue has been resolved. Thank you!---
Separately, I am running into issues with the Coptic calendar. Is there a missing permission on access to the type? To demonstrate, the following works:

use icu::calendar::{Date, DateTime, indian::Indian};

let date_iso = Date::new_iso_date_from_integers(1970, 1, 2).unwrap();
let date_indian = Date::new_from_iso(date_iso, Indian);

But the following does not work:

use icu::calendar::{Date, DateTime, coptic::Coptic};

let date_iso = Date::new_iso_date_from_integers(1970, 1, 2).unwrap();
let date_coptic = Date::new_from_iso(date_iso, Coptic);

With the error:

error[E0423]: expected value, found struct `Coptic`
let datetime_coptic = DateTime::new_from_iso(datetime_iso, Coptic);
                                                           ^^^^^^ constructor is not visible here due to private fields

[Manishearth]

For Coptic you have to call Coptic::new() for now, it's marked as non_exhaustive so it can't be constructed the usual way.

[andrewpollack]

Thanks for the followup @Manishearth !

I went ahead and added the Coptic examples, along with the Buddhist examples. This PR is now in a stable state to review when ready

[Manishearth]

Also i just realized Coptic should not be non_exhaustive, that's a bug. I removed that bit in my other PR.

The thing with Japanese is that it has era data that changes occasionally, so we need to load it. Typically the thing to do in tests is to construct it with a let provider = icu_testdata::get_provider();.

[andrewpollack]

Adjusted the Coptic example with your changes, thanks @Manishearth !

Separately, I'm still running into issues getting the Japanese example working, so I'll work that into a separate PR/discussion. More specifically, getting icu_testdata properly into the Cargo.toml and running in the doctest environment is running into compiler errors that I have not seen before:

thread 'rustc' panicked at 'assertion failed: !new_self_ty.has_escaping_bound_vars()', compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs:1688:9
...
query stack during panic:
#0 [typeck] type-checking `main::_doctest_main_src_japanese_rs_7_0`
#1 [typeck_item_bodies] type-checking all item bodies
#2 [analysis] running analysis passes on this crate
end of query stack
error: internal compiler error: trimmed_def_paths constructed

[Manishearth]

Ah, hmm, we've been getting that error a couple times now, it's rust-lang/rust#96223. It just got fixed, I'll try to get it uplifted to beta.

Make sure that you're importing icu_provider with the "serde" feature as a dev-dependency, that ICE is confusing but it essentially points to a missing trait impl, usually in our case a Deserialize one,