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