Add is_substring (#2018) #831

Job	Run time
docker-master	1h 29m 29s
publish-standalone-dry-run	14s
test-standalone (master, dev, default)	1m 18s
build-js-of-ocaml (master, 4.11.1)	5m 52s
build-wasm-of-ocaml (master, 4.14.1)	17m 42s
standalone-haskell (master)	46m 39s
generated-files (master)	55s
validate-docker-master	21m 15s
test-amd64	2h 16m 25s
deploy-js-wasm-of-ocaml	29s
docker-check-all	1s
	5h 20m 19s

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