[WIP/CR] Batch crafting time reduction

We can now batch craft stuff! And it has a pleasant UI, but *batch* crafting isn't *serial* crafting, at least IRL.

The changes here represent generous and simplistic benefits of batch crafting over serial crafting. (I think the cutoff percentage should probably be higher to be not too generous...)

On the other hand making it simulation-grade would need to take into account: recipe setup time, recipe actual-crafting-time, factor for economies of scale, exact tools available, etc.

I'd like to strike a balance here making the calculation dependent only on the batch size and difficulty of the recipe, in a way that favours batch crafting but doesn't make it a cheat, hence WIP/CR status.

 
We're shooting for having 10 be the highest possible difficulty (and the skill cap).  If any recipes require more than 10 skill, let me know and I'll make the reductions.  (We'll implement a skill cap as soon as mechanics/vehicle-building gets addressed.)