[WIP] Implement the string predicates for link queries

This is pretty gross at the moment, but I thought I'd throw it up for comments on if this is even a reasonable thing to be doing at all. This adds implementations of the various string predicates currently not supported by the newer query expressions and manually builds query objects using them. Since it uses NSString's transformation methods it'd also make it easy to add support for NSDiacriticInsensitivePredicateOption.

@alazier @rrrlasse