Rewrite furnace
* Move furnace related code into furnace.lua
* Move duplicated code into functions
* Rewrite ABM:
	* Easier to follow strcuture (no returns in the middle)
	* No unnecessary calls to get_craft_result
	* Split logic and "visual feedback" (a bit)
	* Fewer calls to meta:set and meta:get
	* Better feedback on the current state of the furnace

Conflicts:
	mods/default/nodes.lua
chg: minetest -> core