Updates (lwd, ocaml-ci), reformat, support cmdliner+logs+fmt

[MisterDA]

The only problem with this PR is that warning and errors messages from Logs will be printed in stderr and disrupt the interface; but perhaps that should serve as an incentive to fix the original problem?

This also supersedes #20.

[tmcgilchrist]

Build failure on ocaml-ci

4 |  (libraries unix capnp-rpc-lwt current_rpc ppx_deriving_yojson.runtime)
                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: Library "ppx_deriving_yojson.runtime" not found.

And the submodule should point to your fixes in let-def/lwd#40.
Other than that the changes look good.

[MisterDA]

Thanks for the review, I've fixed the problems.