Consider having private fields for DataLocale

[Manishearth]

Split off from #5785

After #5988, DataLocale is now in locale_core.

#5875 wanted it to have private fields, but locale fallback (in the locale crate) makes heavy use of these fields, reading from them and mutating them. The fallback iterator steps operate on &mut DataLocale. Migrating this to encapsulated private fields seems to be of little benefit.

The main benefit of private fields is to signpost users away from writing directly to the fields (they should use the Locale/LanguageIdentifier From impls). We're unlikely to change the composition of this type any time soon, the stability benefit is not why we want this. Perhaps we can mark these hidden? Or do nothing?

cc @zbraniecki @sffc @robertbastian