mirage_runtime: restore converters for log_threshold and allocation_policy

[hannesm]

Previously, they were part of the Arg submodule. Now they're in their own Conv module. The reason is that mirage-monitoring uses this value for dynamically adjusting the log level.

[hannesm]

I'm open for suggestions, the code in place (mirage-monitoring) is:

let adjust_log_level s = (* s being a string such as "tcp:error,whateer:debug,something:quiet,*:warning" *)
  let ts =
    List.map
      (fun s -> (fst Mirage_runtime.Arg.log_threshold) s) (* with this PR, it'll be Mirage_runtime.Conv.log_threshold *)
      (String.split_on_char ',' s)
  in
  let* oks =
    List.fold_left (fun acc t ->
        let* acc = acc in
        match t with
        | `Ok l -> Ok (l :: acc)
        | `Error msg -> Error msg)
      (Ok []) ts
  in
  Mirage_runtime.set_level ~default:(Logs.level ()) oks;
  Ok `Empty

[reynir]

I'm not sure it's useful to expose allocation_policy. As far as I can see at runtime it is not useful - it is only used in code generation:

mirage/lib/mirage/mirage.ml

Lines 420 to 426 in 4939ca6

	let pp_pol ~name =
	Fmt.(
	any name
	++ any " = "
	++ any "(match "
	++ Mirage_impl_misc.pp_key
	++ any " with `Next_fit -> 0 | `First_fit -> 1 | `Best_fit -> 2)")

It would however be very nice to have the log_threshold converter back!

[hannesm]

ok, let's merge this, and figure later out if we would like to remove the allocation_policy converter. (there's as well the possibility to use mirage-logs.cli and cmdliner-stdlib -- but I'm afraid they're changing the default values and introduce more dependency complexity, so let's wait at least another release with it).