DONETSK, Ukraine — Uneasy calm returned to the streets of Donetsk on Wednesday after the biggest battle of a pro-Russian separatist uprising in eastern Ukraine, a conflict transformed by the landslide election of a pro-European leader who has vowed to crush the revolt.Government forces killed dozens of rebel fighters on Monday and Tuesday in an assault to retake the airport, which the rebels had seized the morning after Ukrainians overwhelmingly elected Petro Poroshenko as president.Pro-Moscow gunmen have declared the city of a million people capital of an independent Donetsk People's Republic.