Tonight’s guest host, on this new installment, is Halsey. The musical guest also is Halsey, which means she’s pulling double duty – and following in the footsteps of such SNL host-and-musical-performers as Bruno Mars, Britney Spears, Paul Simon, Ray Charles, Justin Timberlake, Taylor Swift, Donald Glover, and the first-ever “I’ll-do-both” host/musician, Desi Arnaz. (Technically, the musical guest for that 1976 episode was his son, Desi Arnaz Jr., but the elder Arnaz returned to his bandleading roots to perform, among other songs, his classic “Babalu.”