module Metadata : sig ... endSpoolable.Metadata.t should be smallish since it is read and written more
frequently than Spoolable.Data.t.
module Data : sig ... endSpoolable.Data.t is where the "real" data lives and it allows for data-specific
load and save functionality.
module Queue : sig ... endQueue.t is an enumerable type that represents the available queues and the
mapping to directory names on-disk.