Last night, the legendary ’60s folk-rock vocal harmonizers Crosby, Stills & Nash were musical guests on Jimmy Fallon’s Tonight Show. This, of course, presented a golden opportunity for Fallon to trot out his uncanny impersonation of Neil Young, the trio’s sometime bandmate, and the four of them sang Iggy Azalea and Charli XCX’s summer-conquering “Fancy,” [...]