followup of #666 (comment)

• @samoht says use Mirage_utils instead of M_util
• also proposes V1 to Mirage (I second that name!)
• we now have mirage.runtime (for keys), and mirage-types (providing pp via M_util), should unify (into mirage.runtime, makes mirage depend on mirage-types (sounds fine!))
• unclear (at least to me, awaiting a brief description from @Drup) what should end up in lib_runtime/mirage_runtime and what in lib/mirage_key