merge: Allow duplicate keys in lhs

[apelisse]

This not only affects merge but also Compare since they use the same algorithm/code.

Duplicates fields in a set/associative-list will now be treated as an atomic entity within that list, and will be entirely owned by the person who made them duplicates or who changed one of the duplicates.

That's the final fix for #234. This comes with extensive testing of all the scenarios I could think of. None of the existing behavior has changed.

/assign @liggitt @alexzielenski

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[liggitt]

/hold to prevent accidental merge until review is done

looks like there are relevant unit test failures

[apelisse]

looks like there are relevant unit test failures

Yeah, the previous PR somewhat broke the union code that is technically not used. Since we might want to backport that change, I don't want to break the API. Not sure how to proceed. I could remove the internal code and just panic while keeping the actual API. Or I could try to fix the union code but that doesn't sound like a useful effort.

[liggitt]

Yeah, the previous PR somewhat broke the union code that is technically not used

wait... those tests passed on that PR though, right?

[liggitt]

oh, these are new tests added in this PR

[apelisse]

Tests are passing now, ready to merge, PTAL!

[liggitt]

/hold cancel /lgtm