Billy Strings, the 30-year-old guitar player, singer-songwriter, and band leader from Michigan, has become a phenomenon in roots music in the past few years. He’s now gone where few bluegrass acts have gone before, headlining arenas, reaching thousands of new fans who are getting their first taste of the tradition through this charismatic and incendiary artist. Will it accrue to the benefit of the genre overall? Craig Havighurst attended Saturday night’s show and came back optimistic.