Draft sketch of ResolvedLocaleAdapter

[sffc]

An example of a potential way to resolve #58

Fixes #3906
Depends on #4606
Depends on #4608

[sffc]

Note to self: we shouldn't implement this, even on DataLocale, due to the same reasons as discussed in #1215. We should instead implement Ord only on the private ResolvedLocaleInfo type.

[sffc]

#4608

[robertbastian]

Another idea is to extend DataProvider with this method:

    /// Query the provider for data, returning an error if the data is not loadable.
    ///
    /// Conceptually returns `self.load(req).map(|_| ())`, but implementations might be able to
    /// provide more performant implementations.
    fn can_load(&self, req: DataRequest) -> Result<(), DataError> {
        self.load(req).map(|_| ())
    }

Using separate return types for separate functionality makes sense, as then the data payload that load returns doesn't have to be optional, reducing fallibility.