IterableDataProvider::supported_locales is a misnomer

[sffc]

Now that we have a more well-established notion of a "supported locale" (#58), we should rename this trait function.

I suggest: iter_locales, and maybe make it return an iterator (if not too hard).

[robertbastian]

I believe this should be

fn supported_requests(&self) -> Result<HashSet<DataRequest>, DataError>

This is because after moving key attributes to the request, supported-locales is not enough to enumerate all supported requests. And if we're using the "requests" phrasing, this will not clash with the notion of supported locales. Also, for performance, I'd like this to return a HashSet instead of an impl Iterator, like we already do internally in datagen.