Adds a secondary material to most comestible items.

Items can have up to two different materials, but comestibles were not making use of this functionality. This resulted in weird stuff like spaghetti with meat sauce being listed as being entirely made of meat, or wheat-based junk food prefering the "junk" material over "wheat", which made it safe for wheat-allergic characters.
This PR makes full use of the material system, adding a secondary material to most comestibles.
Note that it's not 100% complete for gameplay purposes: Since materials are only checked for limited diets, I would be Not Happy if my character was unable to eat a cheeseburger because he was allergic to diaries, since a survivor would just take the cheese off. Some items (mostly sandwiches) were given a few liberties on that sense, since deconstructing them to remove the offending ingredients proved to be very troublesome (ingredients were returned completely fresh and hot, plus they could not match the stuff they were made of when crafted i.e. a sandwich made of hardtack would return bread).

Minorly changes digestion to handle the new materials better: Carnivores still hate veggy and fruit, but their unability to digest grains is handled in the same way as wheat allergy. Also, for some reason, fruit-hating characters would get over their hate if they had bionic digestion, and carnivores couldn't eat dairies: both were confirmed as unintended behavior and fixed.