Republican presidential candidate Vivek Ramaswamy is betting big on digital currency Bitcoin for campaign donations, and he's also unveiling a new proposal to roll back tax restrictions on mining the currency in the U.S. "The thriving Bitcoin universe should actually better empower me to do what I want to do as the U.S. president, which is to stabilize the U.S. dollar as a unit of measurement and put the Federal Reserve back in its place with that as its single mandate," Ramaswamy said in an exclusive interview with CBS News. (He would dispense with the Fed's dual mandate of stabilizing…