Provide a mechanism to list available locales for a given data key.

[gregtatum]

In SpiderMonkey we are asserting in debug builds that our generated locale data are consistent by checking the locales available for different components. ICU supports this through functions like udat_getAvailable. It would be nice to have generic support of this through the provider. I did a quick look through the code and didn't see existing support for it, but I might have missed it.

It seems like with the modularity of the project, finding this out would be useful, especially if using data from different sources.

[sffc]

Action: implement iterable data provider on runtime providers.

[sffc]

Related: #211

I think we need a way for providers to declare what set of languages they support, not at a key level as we currently have, but at the overall provider level.

pub trait LocaleProvider {
    fn iter_locales(&self) -> impl Iterator<LanguageIdentifier>;

    fn supports_locale(&self, &LanguageIdentifier) -> bool;
}

Meanwhile, IterableProvider is and should continue to be a low-level trait with one purpose, to drive the data exporter.

This model means that providers implementing this trait must support the same set of locales for all keys. I think this is an okay and perhaps beneficial constraint. ForkByKeyProvider will not be able to implement this trait, which is fine.

[sffc]

Mostly a duplicate of #58, which is marked as 1.x