#184 and #185 were built in parallel and their candidate types did not
meet. Ranking returned no identity for a candidate, so the winner of a
rank() could not be handed back to Provider::download -- rank() reorders,
so the caller could not recover it by position either.
RankedSubtitle now carries the index of the candidate in the slice it was
given, and Candidate::to_core is the one place the two shapes are mapped:
hash_match against a compared hash, sdh against hearing_impaired, group
against release_group, and the two optional tiebreakers defaulted to sort
last rather than block a candidate.
Miguel Palhas
cdd6133bce