Moscow residents struggle with militiamen outside one of the two banks where they can get foreign currencies. Soviet authorities slashed the exchange rate for convertible currencies nearly five-fold Wednesday, virtually matching prices on the black market. Tens of thousands of workers went on strike Thursday in the Byelorussian capital of Minsk to protest price increases, demand pay raises and call for the resignation of Mikhail Gorbachev. There were reports of strike threats at two enterprises in other cities in Byelorussia, a sign that labor unrest was spreading from striking miners to other parts of the economy.