When novelist Maggie O’Farrell learned that Shakespeare had a son who died just a few years before he wrote “Hamlet” — and that history books barely mentioned it — she wanted to find out more. That led her to write the bestselling novel “Hamnet,” which has now been adapted into a new film starring Paul Mescal and Jessie Buckley. Maggie joins Tom Power to tell us how her Hamnet story came to be, what Shakespeare might have been trying to tell us by naming his play after his son, and why she waited until her own son was past the age that Hamnet was when he died to start writing her novel.