Draft: verify skip_list
erify the specification given in https://nomadic-labs.gitlab.io/coq-tezos-of-ocaml/docs/proofs/skip_list_repr/ by proving Make_is_valid.
erify the specification given in https://nomadic-labs.gitlab.io/coq-tezos-of-ocaml/docs/proofs/skip_list_repr/ by proving Make_is_valid.