AN OLD saying holds that markets are ruled by either greed or fear. Greed once governed cryptocurrencies. The price of Bitcoin, the best-known, rose from about $900 in December 2016 to $19,000 a year later. Recently, fear has been in charge. Bitcoin’s price has fallen back to around $7,000; the prices of other cryptocurrencies, which followed it on the way up, have collapsed, too. No one knows whe