Skin cancer is terrifyingly common—20 percent of Americans will eventually get some form of it, and more than 36,000 women will be diagnosed with mela­noma, the deadliest type, this year. Most of us know the best prevention: Wear sunscreen daily, stay out of the sun during peak hours (that’s 10:00 A.M. to 4:00 P.M., FYI),…