文字とバイト表現の変換をサポート．

signature BYTE =
sig
  val byteToChar : word8 -> char
  val bytesToString : Word8Vector.vector -> string
  val charToByte : char -> word8
  val packString : Word8Array.array * int * substring -> unit
  val stringToBytes : string -> Word8Vector.vector
  val unpackString : Word8ArraySlice.slice -> string
  val unpackStringVec : Word8VectorSlice.slice -> string
end