Migrate to 4.12.0+domains effects implementation(syntax-free effects version)

[bikallem]

This PR updates eio to the OCaml 5.0 supported effects implementation. The migration strategy used is a mechanical one, i.e. effect -> type _ eff += .., match .. with effect .. to match_with.

Note: The PR replicates (1to1) both passing and failing tests in the repo.

Edit: Not strictly necessary but this PR(https://github.com/ocaml-multicore/ocaml-uring/pulls) is needed to make dune b happy since ocaml-uring is a git submodule of this repo.

[talex5]

Thanks - this allows us to enable CI on this repo at last! But the opam metadata needs updating:

- eio_linux -> (problem)
    Rejected candidates:
      eio_linux.dev: Requires ocaml-variants = 4.12.0+domains+effects

[bikallem]

@talex5 looks like we need a .ocamlformat file for the ci to pass. Should I add it? and set the profile to conventional ?

Edit: CI failure -> https://ci.ocamllabs.io/github/ocaml-multicore/eio/commit/95cdeafce2dff6f99d6ffdefdaba61fef7ae59c0/variant/(lint-fmt)

[talex5]

Thanks! I've pushed a few more fixes to your branch. Should be good to merge once CI is finished.