Coinbase Global Inc., the fast-growing exchange at the center of the speculative frenzy in cryptocurrencies, is expected to go public this week at a staggering valuation of about $100 billion. That’s more than the venerable New York Stock Exchange and Nasdaq Stock Market combined -- for a company that didn’t even exist a decade ago. If all goes according to plan, Wednesday’s scheduled direct listing on Nasdaq will cement Coinbase’s position as the Big Board of the U.S. crypto scene and a potent symbol of the risks and rewards of the new era of digital money.