Reworking filter provider

[robertbastian]

There are a few things here:

• The filters should work on DataIdentifierBorrowed instead of DataRequest; metadata should not be relevant for filtering
  • This implies a rename from RequestFilterDataProvider to FilterDataProvider
• previously the provider implementations returned DataErrorKind::Filtered for filtered ids, however this does not work with fallback. They should return DataErrorKind::IdentiferNotFound; to a caller (like the fallback adapter) it should be opaque whether the pipeline contains a filter or not
• filter_by_langid_allowlist_strict is marked as "will be replaced with a smarter algorithm for locale filtering". This smarter algorithm now exists, so this should be deleted (fixes Add smarter locale filtering in DataExporter #834)
• The Filterable blanket trait adds .filterable to every type in this crate's rustdoc, and to every type in dev docs. I replaced it with a normal constructor.

[Manishearth]

seems straightforward

[sffc]

OK