Better opaque/abstract value inspection

[hackwaly]

I tested utop example. It displays "some_values" perfectly. I don't know how it works. Maybe we can learn how it achieved.

https://github.com/ocaml-community/utop/blob/8cc563282597abdb8f5cca64df41166c5ebca6b5/examples/interact/test_program.ml#L1-L11

[sim642]

Is it really abstract/opaque there though?

[hackwaly]

I think it is.

type value = V : string * _ -> value

val interact
  :  ?search_path:string list
  -> ?build_dir:string
  -> unit:string
  -> loc:(string * int * int * int)
  -> values:value list
  -> unit
  -> unit