Rework metal/stone techs to cost Food and Stone.
Food/Wood techs now cost Wood and metal (with food techs being slightly less expensive as they provide less bonus). Metal/Stone techs now cost Food and Stone instead of Wood and Metal, reducing the load on those resources, and making Stone slightly more critical.
Initial work by: Nescio
Accepted By: chrstgtr, Valihrant
Differential Revision: https://code.wildfiregames.com/D3704