add Script#cast_to_string

  fixes:

    tx      = Bitcoin::Protocol::Tx.from_json(File.read('61a078472543e9de9247446076320499c108b52307d8d0fafbe53b5c4e32acc4.json'))
    prev_tx = Bitcoin::Protocol::Tx.from_json(File.read('5342c96b946ea2c5e497de5dbf7762021f94aba2c8222c17ed28492fdbb4a6d9.json'))
    tx.verify_input_signature(0, prev_tx) == true