Obi integration in opam-check-all

[kit-ty-kate]

For now kit-ty-kate/opam-check-all uses the obi library to the list of maintainers per packages but nothing more. To be able to use the obi fully in opam-check-all here what I would need:

• Full logs: opam-check-all needs access to the full log of the opam build. It is "common" to see errors being displayed outside the "opam error square" either because it was too long, because there is not enough informations or because one of the dependencies failed (and is not retrievable outside of this particular combination, e.g. some particular mix of depopts or depexts makes it fail in a way it wouldn't otherwise). Access to the latest logs through a link seems doable (for 3 versions of OCaml, it takes about 2Go of disk space)
• Obi.Index.params consistency: opam-check-all needs some kind of hint to know which versions of OCaml are available (are they hardcoded ?), whether x86_64 builds have been successfully done (can I be assured they are always available ?) and which distributions have been built. It might be good to have some kind of header in the index file to have this information available right away.

Concerning the distributions issue: I guess I'd have to make a ranking at some point. opam-check-all uses Debian Unstable as it seems to be the distribution with the most external package available (e.g. latest versions of LLVM, latest version of SDL used by tsdl, and so on…).
However if anybody have some hints about which distributions would be the most suited to display result by default (not Debian Stable in my opinion), it would be much appreciated.

[avsm]

I like the idea of the aggregate header indicating the total set of versions and architectures available. Related to #6 to make rendering easy for clients. Also related to #9 because Obi needs to use ocaml-version instead of hardcoding OCaml compiler versions.