merge Group.Scalarmult into MontgomeryLadder, use in x25519_base (#1843) #422

Job	Run time
docker-master	1h 37m 16s
publish-standalone-dry-run	17s
test-standalone (master, dev, default)	59s
build-js-of-ocaml (master, 4.11.1)	13m 49s
generated-files (master)	3m 6s
build-wasm-of-ocaml (master, 4.14.1)	13m 41s
standalone-haskell (master)	55m 2s
test-amd64	2h 22m 7s
validate-docker-master	35m 27s
deploy-js-wasm-of-ocaml	33s
docker-check-all	11s
	6h 2m 28s

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