In March, literary heavyweights Kazuo Ishiguro and Neil Gaiman — a Nobel laureate, and the beloved author of “American Gods,” “Sandman,” and “Good Omens,” respectively — convened at an independent bookstore event to discuss genre and science fiction. They arrived at twin conclusions: one, that rigid genre distinctions between literary works promote an unproductive and false hierarchy of worth, and two, that the 21st century is a very tricky time to attempt to define “science fiction” at all. Gaiman said that he increasingly feels genre “slippage where science fiction is…