Batteries user guide : BatDigest

`sig
type t = string
val string : string -> BatDigest.t
val substring : string -> int -> int -> BatDigest.t
val file : string -> BatDigest.t
val to_hex : BatDigest.t -> string
val from_hex : string -> BatDigest.t
val channel : BatIO.input -> int -> Digest.t
val output : 'a BatIO.output -> BatDigest.t -> unit
val print : 'a BatIO.output -> Digest.t -> unit
val input : BatIO.input -> Digest.t
val compare : BatDigest.t -> BatDigest.t -> int
end`