“Hadestown,” with its free-form jamboree vibe, its spontaneous swirl of musicians and dancers onstage, is as much a happening as it is a musical. It’s a show about ancient mythology — it tells the story of Orpheus and Eurydice, which it interweaves with the tale of Hades and Persephone. That sounds daunting, but “Hadestown,” with its book, music, and lyrics by Anaïs Mitchell, succeeds in making those myths feel timeless, which of course they are. It’s a show about love, but more than that it’s a show about faith.