1.15.1

CHANGES:

Opam CI fixes by @MisterDA (#54):

• update to Dune 2.9 to fix lint warning;

  (lint) (failed: Warning in sha.1.15: Dubious use of 'dune subst'. 'dune subst' should always only be called with {dev} (i.e. ["dune" "subst"] {dev}) If your opam file has been autogenerated by dune, you need to upgrade your dune-project to at least (lang dune 2.7).)

• define Bytes_val ourselves before OCaml 4.06;
• add setup-ocaml GitHub action to catch mistakes earlier.