an <a href="#InventoryRange">InventoryRange</a> with the slots at the given indexes
RangeError if an invalid range or index is given
auto inventory = new Inventory(4); inventory[2] = Slot(new Items.Apple(), 1); assert(inventory[2..$] == [Slot(new Items.Apple(), 1), Slot(null)]);
Gets the slots in a specific range.