This PR proposes to remove the function Domain.at_each_spawn.

One major issue with Domain.at_each_spawn is that it is impossible to predict in which context the registered function will end up being called.

A good illustration of the issue at hand is the use of Domain.at_each_spawn in the threads library in order to initialize the thread machinery. This approach breaks down if the order of the initialization is incorrect as described in #11434. Consider for instance, a module Early_hook registering a function with Domain.at_each_spawn

(* early_hook.ml *)
let f = ref ignore
let () = Domain.at_each_spawn (fun () -> !f ())

and the following main function:

(* main.ml *)
let print_self () =
  Printf.printf "self=%d\n" (Thread.id (Thread.self ()))
let () =
  Early_hook.f := print_self;
  for i = 0 to 50 do
    Domain.join (Domain.spawn ignore)
  done

With this setup, any program compiled with early_hook linked before the systhreads library crashes because the function registered by early_hook calls Thread.self before the thread machinery initialization.

The first commit in this PR fixes this issue by adding a new C hook caml_domain_initialize_hook for the systhreads library which is run during domain initialization just before the registration with the stop-the-world mechanism. (I am not sure of the right location for the hook during initialization). The second commit adds the example described above as an ephemeral test.

The two last commits removes the use of Domain.at_each_spawn in Format by moving the registration of the Domain.at_exit functions to the initialization of the by-domain stdout and stder formatter.

Finally, the last commit completely remove Domain.at_each_spawn function since the function seems more harmful than helpful in its current state. It is thus probably better in term of forward compatibility to remove it.

Close #11434