Op.Wait_filesWaiting on files.
FIXME now that we have Futs maybe we can maybe get rid of this.
val v_op :
id:id ->
mark:mark ->
created:B0_std.Mtime.span ->
?post_exec:(op -> unit) ->
?k:(op -> unit) ->
B0_std.Fpath.t list ->
opv declares a wait files operation, these are stored in reads.
val v : unit -> tv constructs a bare wait files operation.