tezos-shell-services: Add support for OCaml 4.12
Manually testing the MR
$ opam switch create 4.12.0+trunk
$ opam remote add alpha git://github.com/kit-ty-kate/opam-alpha-repository.git
$ opam install --deps tezos-shell-services
$ dune build src/lib_shell_services/tezos-shell-services.install