def to_wif
      version = Bitcoin.chain_params.privkey_version
      hex = version + priv_key
      hex += '01' if compressed?
      hex += Bitcoin.calc_checksum(hex)
      Base58.encode(hex)
    end
def to_wif
      version = Bitcoin.chain_params.privkey_version
      hex = version + priv_key
      hex += '01' if compressed?
      hex += Bitcoin.calc_checksum(hex)
      Base58.encode(hex)
    end
export private key with wif format