def to_mnemonic(entropy)
      raise ArgumentError, 'entropy is empty.' if entropy.nil? || entropy.empty?
      e = entropy.htb.unpack('B*').first
      seed = e + checksum(e)
      mnemonic_index = seed.chars.each_slice(11).map{|i|i.join.to_i(2)}
      word_master = load_words
      mnemonic_index.map{|i|word_master[i]}
    end
def to_mnemonic(entropy)
      raise ArgumentError, 'entropy is empty.' if entropy.nil? || entropy.empty?
      e = entropy.htb.unpack('B*').first
      seed = e + checksum(e)
      mnemonic_index = seed.chars.each_slice(11).map{|i|i.join.to_i(2)}
      word_master = load_words
      mnemonic_index.map{|i|word_master[i]}
    end
generate mnemonic words from entropy.
 @param [String] entropy an entropy with hex format.
 @return [Array] the array of mnemonic word.