rem(container): sizing options

This enables the `Layout` struct to be packed (as well as the
`Properties` struct) which should further reduce the memory footprint.

examples/container.zig

@@ -41,10 +41,6 @@ pub fn main() !void {
             .gap = 1,
             .direction = .horizontal,
             .padding = .vertical(1),
-            .sizing = .{
-                // .width = .{ .fixed = 700 },
-                // .height = .{ .fixed = 180 },
-            },
         },
     });
     try box.append(try App.Container.init(allocator, .{