Digest

`sig
type t = string
val compare : Digest.t -> Digest.t -> int
val equal : Digest.t -> Digest.t -> bool
val string : string -> Digest.t
val bytes : bytes -> Digest.t
val substring : string -> int -> int -> Digest.t
val subbytes : bytes -> int -> int -> Digest.t
external channel : Stdlib.in_channel -> int -> Digest.t = "caml_md5_chan"
val file : string -> Digest.t
val output : Stdlib.out_channel -> Digest.t -> unit
val input : Stdlib.in_channel -> Digest.t
val to_hex : Digest.t -> string
val from_hex : string -> Digest.t
end`