Add mcl_util.is_fuel

mods/CORE/mcl_util/init.lua

@@ -188,3 +188,9 @@ function mcl_util.get_first_occupied_inventory_slot(inventory, listname)
 	end
 	return nil
 end
+
+-- Returns true if item (itemstring or ItemStack) can be used as a furnace fuel.
+-- Returns false otherwise
+function mcl_util.is_fuel(item)
+	return minetest.get_craft_result({method="fuel", width=1, items={item}}).time ~= 0
+end