OCaml library : Digest.S

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