Store_migration.MakeFunctor to create and register a migration.
module S_from : Store_sig.Smodule S_dest : Store_sig.Smodule Actions : 
  MIGRATION_ACTIONS
    with type from_store := Mavryk_layer2_store.Store_sigs.ro S_from.t
     and type dest_store := Mavryk_layer2_store.Store_sigs.rw S_dest.tval migrate : 
  Metadata.t ->
  storage_dir:string ->
  index_buffer_size:int ->
  unit Mavryk_base.TzPervasives.tzresult Lwt.tMigration function for the store located in storage_dir.