- Specialized structure for tags: we use a specific structure for the
  tags, where we handle the fact that some tags that are known can only
  apear once. We also try to avoid allocations if we have only one value
  for the tag.
- SmallVec to store languages and authors and owners in karas and
  playlists, because they won't be too many so try to avoir allocations
  as much as possible.
- We clean up a bit the code, and use something else than a macro to
  implement reading and checking a folder of hash-verified files…