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