irmin-fs: Eio backend

[art-w]

Follow-up on #2149 to fix the Eio backend of irmin-fs.unix:

• The effects to access the eio capabilities had to go, as they break as soon as we spawn fibers which will escape the handler scope
• Rather, the eio capabilities are passed through the irmin Conf as this is the least invasive solution that we found... The main issue is that irmin-cli expects to be able to deserialize the conf values from the command-line / config file: the underhanded solution to this is to parametrize the conf Spec by the required eio capabilities. Cons: we technically loose spec uniqueness per backend, but this didn't seem to be used anyway other than for sanity checking irmin?

WDYT of renaming irmin-fs.unix to irmin-fs.eio?