- eliminates nearly identical code between 'gather' and 'combineGather'
* Normal gather updates by assigning the result of the binary operation.
* Combine gather updates by using a binary operator that modifies
its first parameter in-place
By-product of this refactoring are these new variants:
listGather(), listGatherReduce()
mapGather(), mapGatherReduce()
that mirror the previously existing ones
listCombineGather(), listCombineReduce()
mapCombineGather(), mapCombineReduce()
except that they use the 'regular' binary operator
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