Add methods to iterate exemplar characters

[lionel-rowe]

Adds public code_point_ranges and strings methods to ExemplarCharacters to allow iterating through the single- and multi-character entries, respectively:

• code_point_ranges returns a CodePointRangeIterator (existing struct)
• strings returns a StringIterator, a new struct that iterates via passing a DiplomatWrite buffer to its next method, writing a null-char-terminated string for Some(string) and an empty string for None (thus differentiating Some("") -> "\0" from None -> "", though in practice Some("") should never occur for exemplar chars). Not sure if there's a better way of iterating strings across the FFI boundary.

Thus for hu locale and Main set:

# `code_point_ranges`
'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'r', 's', 't', 'u', 'v', 'z', 'á', 'é', 'í', 'ó', 'ö', 'ú', 'ü', 'ő', 'ű'
# `strings`
"ccs", "cs", "ddz", "ddzs", "dz", "dzs", "ggy", "gy", "lly", "ly", "nny", "ny", "ssz", "sz", "tty", "ty", "zs", "zzs"

I also added a tab to the WASM demo to illustrate how these can be used from JS/TS.

[Manishearth]

Mostly looks good! Curious: what are you using ICU4X-JS for? So far we've been getting C++ users, excited to see some uptake in the JS backend!

[Manishearth]

issue: Diplomat supports iterators, I suggest tacking on #[diplomat::attr(auto, iterator)] and having this return an Option<()>.

We should probably make sure all other iterable APIs in icu_capi use iterators too.

[Manishearth]

we shouldn't add a generic stringiterator type IMO, it's going to do expensive intermediate allocations.

[Manishearth]

issue: this should reference a concrete type, and borrow from it

[Manishearth]

issue: these two should not collect, Diplomat can return borrows. This should return a Box<ExemplarCharactersStringIterator<'a>>

Ideally there's no dynamic dispatch, if we need concrete iterator types in ICU4X (where we have `impl Trait) we should add them.