Bitcoin is crashing — upward. It just passed $28,000 and shows no signs of stopping.The digital currency has a market value north of $500 billion. Think Bitcoin is just a fad? It’s worth more than Visa or Mastercard. Or Walmart.Bitcoin passed $20,000 for the first time just 11 days ago. Now it’s knocking at $30,000’s door.Its rapid rise has been remarkable — or unbelievable, depending