You've got your gourmet on-the-run storefronts and your ethnic takeouts. Yet another key element in the American diet mix is distinctly frozen. But why buy Swanson's and Stouffer's when you can get the same food that some restaurants buy and reheat? What started out in the 1890s as a family business with a horse-drawn cart has emerged as one of the nation's top 25 wholesale distributors. Allen's retail store features a freezerful of everything from lasagna to apple strudel to dill puff appetizers to giant bags of frozen broccoli soup. Another reason to visit is the minifarm out back in the parking lot, replete with sheep, chickens, goats and ducks.