DROP THE KNIFE! You've got your egg slicer, your apple slicer, your pineapple slicer, now you can have your banana slicer. . . . William Sheffield, a retired Orange County Superior Court judge, thought it up one day while walking down a street in Hong Kong, where he was living. . . . Back in Orange County, he tinkered. He fiddled. He shaped it like a banana. He sent prototypes to Queen Elizabeth II and President and Mrs. Bush. The Bushes called it "special" and the Queen's letter answerer thanked him for "this great invention." Vons is selling it now, and Bloomingdale's, which knows a trend when it sees one, will jump aboard soon. Price will be $2.29 to $2.89 depending on where you buy it.