complete landmarks support

[redianthus]

(this is a follow-up on #871)

TODO:

• make sure domainpc actually crashes when there are too many domains (cc @hra687261) EDIT: now it is crashing when it should not (saying there are 0 cores available when there are many) EDIT: it now crashes properly, but now some tests are failing (saying All OK instead of finding a bug, as if nothing was happening, but it is not reproductible outside of dune...) EDIT: it works
• change the API of domainpc to be able to get the n in spawn_n (useful because when we pass it None we can't guess n), it should be a function available before calling spawn_n, and not a number given after calling spawn_n
  • Done in https://github.com/OCamlPro/domainpc through get_available_cores
• I think we don't want to isolate the main domain when running in "performance" mode, this should be done only in landmarks, if domainpc can offer this in its API I can take care of implementing it in Owi
  • Now it is not done by default we have to explicitely call isolate_current
• publish a release of domainpc in opam/nix (@hra687261, can you take care of opam and I'll do nix?)
  • WIP: [new release] domainpc (0.1) ocaml/opam-repository#29223
• wait for landmarks to be upstreamed and updated in opam/nix
  • WIP: PR created Support using landmarks with OCaml 5 domains LexiFi/landmarks#46
• add a flag --no-worker-isolation which uses Domain instead of Domainpc (it'll allow to use more workers than the number of physical cores)