Add `distinct-by` and `distinct` reducers utilities.
Clean up output paths on job failure.
Support task tuple collections as lazy sequences.
Drop test job poll interval to speed up tests.
Correct handling of `reduced` in tuple source reduction.
Explicit local tuple sources; dseqs as local tuple sources.
Update to most recent Abracad release.
Improve examples.
Update some TODOs.
Update news file with new features.
Release version 0.4.1.