Use `dune` when installing via OPAM

[Leonidas-from-XIV]

This uses dune to build and install the generated artifacts when installed via OPAM using the default dune commands that the dune documentation suggests.

I am not sure this has been proposed before, I couldn't find anything in that regard.

Dune also supports a split between development mode and release mode where warnings are not fatal, hence:

Closes #19

[dra27]

Dune also supports a split between development mode and release mode where warnings are not fatal, hence:

This isn't true, because of https://github.com/ocaml/num/blob/703e1f883c8728ed0abaed6e2d5cdd3fc7e3cb46/src/dune#L19

[dra27]

This PR as it stands would break the installation guarantees for num on 4.06-4.14. Wherever possible, the num library should be installed to ocaml's lib directory.

The dune files also contain some minor errors which prevents their being immediately switched to as an installation mechanism.

However, I have some commits which in addition to #29 would improve the situation and we could look at dune being a depopt for num in the same way as, say, opam-file-format.