Introduce backwards compatible infrastructure for parallelism #13140

Job	Run time
regression (macos-13, ocaml-variants.4.14.2+options,ocaml-option-flambda)	24m 56s
regression (ubuntu-22.04, ocaml-variants.4.14.2+options,ocaml-option-flambda)	8m 21s
extraction	-1s
	33m 16s

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