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

src/pocketmine/inventory/AnvilInventory.php

@@ -49,6 +49,7 @@ class AnvilInventory extends ContainerInventory{
 	}
 
 	/**
+	 * This override is here for documentation and code completion purposes only.
 	 * @return FakeBlockMenu
 	 */
 	public function getHolder(){