Move to eio

[clecat]

This pull requests goal is to finalize to move from lwt to eio within the irmin project, it should be merged and reviewed once we are satisfied by the state of the eio branch and most of the PRs pointing to it are merged.

It will also allow to keep an eye on the status of the CI.

[samoht]

@patricoferris @lyrm @art-w @ElectreAAS @clecat -- are you happy to merge this in main ? Any blocker?

[clecat]

I do not see any major blockers

[lyrm]

I made a list of things that needed fixes here . This is clearly outdated, but it's likely that not everything has been done. A lot of it is small tasks (updating examples and documentation), and it was decided that it would be done before merging to main, if I remembered correctly.

[zshipko]

I think https://github.com/mirage/irmin/pull/2318 should also be included