Provide access to a Spoolable.Data.t. Data_file.t functions as a "handle" to
the underlying data so the user can choose when to read a Spoolable.Data.t.
val path : t ‑> stringval load : t ‑> Spoolable.Data.t Async.Deferred.Or_error.tval save : t ‑> contents:Spoolable.Data.t ‑> unit Async.Deferred.Or_error.tval stat : t ‑> Async.Unix.Stats.t Async.Deferred.Or_error.t