OTHER: Use xmmsv_(de)serialize to save/load collections.
OTHER: Add versioning to collection on-disk format.
OTHER: Change collection format in sqlite conversion tool.
FEATURE(2524): Bump src/lib/s4

* src/lib/s4 a9aff53...48487d9 (3):
  > OTHER: Add an order-builder to the s4_resultset_sort API.
  > OTHER: Remove unused _oplist_is_empty function.
  > OTHER: Add void arg to param-less funcs.
FEATURE(2524): Add field coalescing to Order collection.
FEATURE(2524): Accept list of strings for field attr of Order coll.
FEATURE(2524): Use coalescing sort in cli search.
OTHER: Fix broken medialib test.

Ordering the union by id breaks the component ordering
which is the ordering the test tries to exercise.