Mariah Carey’s reputation for five-octave vocal gymnastics can sometimes overshadow the fact she’s also one of the most prolific songwriters in contemporary pop and R&B. Be it soulful early-’90s singles like “Emotions,” eternal-sunshine bops like “Fantasy,” or modern slow-jam classics like “We Belong Together,” Carey’s work is both trendsetting and timeless, as natural a fit for new-school girl groups like Allure as for legends like Daryl Hall.