v0.14.0

Implements spec v0.44: reverse-reference queries in the indexing service (Addendum A).

New:

• The index collects every x-ref reference of typed documents by §6.5's walk, with the
  schema revision each document pins (no x-index needed), into a new refs table.
• ?ref=/r/{ns}/{name} finds the documents that reference a resource in any form;
  …/rev/{id} only those pinned to that revision; …%23{entry} only those naming that
  entry. It combines with the other filters, and hits carry refs: [{ path, ref }].
  Malformed or repeated ref is 400.
• An index database from before v0.44 is rebuilt once at startup, so its references are
  collected.
• GET / answers "spec": "0.44".

Full Changelog: v0.13.0...v0.14.0