Refactor live neighborhood finder

- Create NeighborFinder interface to replace NeighborhoodFinder
- Implement live and snapshot-based neighbor finders
Get the user-user tests running again
Measure the per-user test time.
Record per-user event counts
Bump vectorz dependency version
Improve FunkSVD cache locality

Use per-feature scratch vectors to improve the cache locality of building the FunkSVD recommender.
Improve docs and clarity for FunkSVD cache fixes
Refine types to help optimizer

Use Vector instead of AVector to maybe help the optimizer inline methods.
Clean up FunkSVD model builder

- Just use Collection interface
- Delete unused method
Merge pull request #566 from elehack/feature/elehack-uu-fast

Algorithm performance improvements