Add is_substring #829

Job	Run time
docker-master	1h 29m 42s
publish-standalone-dry-run	18s
test-standalone (master, dev, default)	1m 22s
build-js-of-ocaml (master, 4.11.1)	6m 26s
build-wasm-of-ocaml (master, 4.14.1)	16m 34s
generated-files (master)	23m 5s
standalone-haskell (master)	47m 35s
test-amd64	2h 15m 54s
validate-docker-master	2m 27s
deploy-js-wasm-of-ocaml	29s
docker-check-all	1s
	5h 23m 53s

Provide feedback