port to multicore-OCaml

[UnixJunkie]

if a volunteer shows up...

[UnixJunkie]

If there was a version taking advantage of ocaml-5 features, it would allow people to easily switch between fork-based parallelism and thread-based parallelism (just by changing the parmap version or branch that they install).
Doing this might help to show use cases where ocaml-5 runtime could (should?) be improved.