At the Armory, I found “Illinoise” not only sentimental but downright whimsical. It was intriguing to see three singers walk out on stage wearing enormous fairy wings (costumes by Reid Bartelme and Harriet Jung). Only later did I realize that they were moth wings, because “Illinoise,” with its book by Justin Peck and Jackie Sibblies Drury, is a coming-of-age story. The dancers on stage can be seen as human caterpillars that blossom into human moths. “Illinoise” could be called “Metamorphosis,” but the songs come from Sufjan Stevens’ 2005 concept album, “Illinois.” The switch from that original title to “Illinoise” is not a typo. Chalk it up to the change being another metamorphosis, using the spelling seen on the album’s stylized cover, which read “Come on feel the Illinoise.” Beyond “sentimental,” the other word to describe “Illinoise” is “precious.”