Add `split_result` #651

Job	Run time
format (5.1)	2m 31s
build (macos-latest, 5.1)	2m 20s
build (ubuntu-latest, 4.08)	2m 1s
build (ubuntu-latest, 5.1)	2m 49s
build (ubuntu-latest, 4.14)	1m 51s
build (ubuntu-latest, 5.2)	2m 31s
build (ubuntu-latest, ocaml-variants.5.0.0+options,ocaml-option-bytecode-only)	3m 20s
	17m 23s

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