def to_seed(mnemonic, passphrase: '')
      to_entropy(mnemonic)
      OpenSSL::PKCS5.pbkdf2_hmac(mnemonic.join(' ').downcase,
                                 'mnemonic' + passphrase, 2048, 64, OpenSSL::Digest::SHA512.new).bth
    end
def to_seed(mnemonic, passphrase: '')
      to_entropy(mnemonic)
      OpenSSL::PKCS5.pbkdf2_hmac(mnemonic.join(' ').downcase,
                                 'mnemonic' + passphrase, 2048, 64, OpenSSL::Digest::SHA512.new).bth
    end
generate seed from mnemonic
 if mnemonic protected with passphrase, specify that passphrase.
 @param [Array] mnemonic a array of mnemonic word.
 @param [String] passphrase a passphrase which protects mnemonic. the default value is an empty string.
 @return [String] seed