Use SplFixedArrays in inventory, added more typehints and cleaned up some duplicated code

src/pocketmine/inventory/ChestInventory.php

@@ -55,6 +55,7 @@ class ChestInventory extends ContainerInventory{
 	}
 
 	/**
+	 * This override is here for documentation and code completion purposes only.
 	 * @return Chest
 	 */
 	public function getHolder(){