Before the 57th Street supertall boom began, the corridor had relatively few icons. But among those few, Sheldon Solow’s 9 West 57th Street stands as one of the most monumental buildings in all of Midtown, and also one of the most expensive. Now, the developer is building a smaller residential tower next door, at 7 West 57th Street, and the project’s designers, Hill West Architects, have sent along a rendering of what it will soon look like.