Mariah Carey, the biggest-selling female recording artist of the 1990's, is ringing out the decade with Rainbow, her first album of all-new material since 1997's quadruple-platinum Butterfly. According to Billboard (October 16, 1999), Rainbow is "a seamless synthesis of the pure-pop sound that distinguished her early efforts and the streetwise rhythms inherent in her more recent recordings." And Mariah's worldwide legions of fans -- who've bought more than 120 million copies of her albums and singles since her self-titled debut in 1990 -- are bound to agree