Collection diffing. (#1)

* Collection diffing.

* The move elimination passes now emit correct indices.

* Update the docs on mutation offsets. Improve the correctness of the reproducibility tests.

* Update all test cases to test for reproducibility. Add a new random data test case.

* Fix a couple of malformed iteration ranges.

* Revert the removal of the position invariant guarantee of `Changeset.mutations`.

* Elaborate on `moves` not necessarily representing all moves.

Author: andersio

FlexibleDiff.xcodeproj/project.pbxproj

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