The road to hell is paved with heavenly music in “Hadestown,” the exuberant, exhilarating new musical that stands tall among a season of mostly (so far) lackluster Broadway competition. A retelling of the myth of Orpheus and Eurydice, the show features a book and score by the singer-songwriter Anaïs Mitchell that ranks among the most adventurous and accomplished to be heard on Broadway in recent years. And the production, developed and directed by Rachel Chavkin, superbly showcases both Mitchell’s work and a superlative cast, accompanied by an orchestra — perhaps more aptly called a band — so seamlessly integrated into…