Publish Z_mlgmpidl module on opam

[sim642]

As pointed out in #118, the Z_mlgmpidl module present here can be used, but it's not built or packaged on opam. Since it works, it shouldn't hurt to also publish it on opam.
It would be useful for Apron users (e.g. Goblint, MOPSA, Frama-C, etc.), because the alternative is to perform the conversions through string, which isn't very pleasing, or float, which defeats the point of big integers.

Of course the module depends on mlgmpidl, so it wouldn't make sense for the module to be part of the "zarith" package, which would have to have an additional dependency for no good reason. Therefore, that single module could be published as a separate package like "zarith-mlgmpidl", which could still be defined in this repository and depend on both zarith and mlgmpidl.

[antoinemine]

Could we use optional dependencies in opam?

[sim642]

I suppose that should work as well, if the build is made to detect the presence of mlgmpidl.
I think there are also opinions that optional dependencies are not the best to have, this is often linked: http://rgrinberg.com/posts/optional-dependencies-considered-harmful/.

Whichever way the module is made available without having to copy it works for me at least, but maybe some people have a preference about the setup of such central package.