“For decades, when a Native American woman has been assaulted or raped by a man who is non-Indian, she has had little or no recourse. Under long-standing law in Indian country, reservations are sovereign nations with their own police departments and courts in charge of prosecuting crimes on tribal land. But Indian police have lacked the legal authority to arrest non-Indian men who commit acts of domestic violence against native women on reservations, and tribal courts have lacked the authority to prosecute the men.”