Add comparison keys to plan string for in-union

[alecgrieser]

The comparisonKeyFunction is currently not included in the plan representation of InUnion plans. This can make it hard to inspect bugs that could be a result of an in-union improperly merging results from different legs of a union. This does make the plan representation a little more verbose, though hopefully manageably so. It should be less intrusive, for example, than a regular union, which puts the comparison key between each leg of the union (unless the comparison is by primary key).