affeq: optimize sparse affeq join #13627

Job	Run time
regression (ubuntu-22.04, ocaml-variants.4.14.2+options,ocaml-option-flambda)	8m 57s
regression (macos-13, ocaml-variants.4.14.2+options,ocaml-option-flambda)	22m 6s
extraction	-1s
	31m 2s

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