testable for seq and contramap (for testable) #479

Job	Run time
build (ubuntu-latest, ., true, 5.x)	5m 11s
build (ubuntu-latest, ., true, 4.14)	4m 43s
build (macos-latest, 4.14, ., true)	7m 10s
build (windows-latest, 4.14, alcotest alcotest-js alcotest-lwt alcotest-mirage, false, alcotest.o...	5m 36s
build (ubuntu-latest, 4.08, alcotest alcotest-js alcotest-lwt alcotest-mirage, false, alcotest.op...	2m 58s
build (ubuntu-latest, 4.09, alcotest alcotest-js alcotest-lwt alcotest-mirage, false, alcotest.op...	2m 20s
build (ubuntu-latest, 4.10, alcotest alcotest-js alcotest-lwt alcotest-mirage, false, alcotest.op...	2m 19s
build (ubuntu-latest, 4.11, alcotest alcotest-js alcotest-lwt alcotest-mirage, false, alcotest.op...	2m 29s
build (ubuntu-latest, 4.12, alcotest alcotest-js alcotest-lwt alcotest-mirage, false, alcotest.op...	2m 29s
build (ubuntu-latest, 4.13, alcotest alcotest-js alcotest-lwt alcotest-mirage, false, alcotest.op...	2m 35s
	37m 50s

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