Consider a tale that’s (unfortunately) as old as time: a teenage girl falls in love with a seductive older man, abandons her previous life, and destroys herself in the process. In Caroline Leavitt’s new novel “Cruel Beautiful World,” the story is retold yet again, with comparisons to the women of the Manson family serving as shading for its sympathetically-drawn portrait of a young woman’s ill-fated relationship and its heartbreaking consequences.