New Comparator/Hopper/Furnace Interactions

All three slot processors (smoker, blast furnace, furnace) should be simplified to have an input (fuel, whatever they are 'cooking') and an output (whatever they've cooked).

This would let you use other sides of the furnace for comparators. Specifically you should be able to compare fuel and fuel burn time to the # of items to smelt (so you can only put fuel in when there are enough items to fully use the fuel) and so that you can leave one item in the furnace (so you can take out the last item to get XP).  I'm thinking maybe also change it so if it is 'locked' it can have fuel and something to smelt in it at the same time without activating rather than by locking out new items. This would let you compare the input and fuel slots to toggle the furnace on.

That's the simplest way I can think to accomplish being able to do that within the normal redstone/gui framework, although if someone has a better solution let me know.  I'm thinking of a smelting system that can always fully uses whatever fuel it has and can be toggled to leave one item in for XP while still accomplishing everything it can regularly do.  Right now the comparator doesn't work in a particularly useful way with the furnace, and I think any of the things you can use it for now could still be replicated with this system.