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Add a row_reduce primitive and migrate sum onto it #563

Job	Run time
build (windows-latest, Release)	9m 55s
build (windows-latest, Debug)	9m 15s
build (ubuntu-latest, Debug)	22s
build (macos-latest, Release)	29s
build (macos-latest, Debug)	23s
build (ubuntu-latest, Release)	34s
	20m 58s