Move Irmin_pack.Io to eio

[clecat]

Make the Io module of Irmin-pack compatible with eio.
This work involves using the Eio related functions for all io operations.
Most of the work relies on propagating the eio switch that will be necessary in order to open the files.

[art-w]

Thanks a lot for all the hard work! I've rebased your PR on top of #2316 and changed a few things to pass the Eio capabilities through the Irmin Conf/Spec in the same way as for the irmin-fs eio backend. I'm leaving this PR as a draft for now as I saw a couple of (* TODO *) left in the diff and things we could still simplify :)

[cuihtlauac]

That's a partial review, I haven't looked at bench, examples and test. I don't get the details, but it looks fairly good. Some cosmetic remarks along.

[samoht]

What's the status of this PR. @lyrm and @zshipko are you happy to merge?

[zshipko]

I think I'm in favor of merging this if no one else objects and following up with some additional PRs around synchronization (like #2281) and testing

[lyrm]

Sounds good to me.

[samoht]

Merged !

Thank you, everyone, for your hard work on this (and thanks to @clecat for pushing it through). Great job!