release procedure

git checkout -t -b vM.N.O
topkg tag M.N.O
dune subst
ocaml tools/gen_version.mlt src/Version.ml M.N.O
git add src/Version.ml
git commit -a -m vM.N.O
topkg tag M.N.O -f
git push origin refs/heads/vM.N.O refs/tags/M.N.O
opam publish ocamlformat.opam