Remove MaterialInfo consts

[quietust]

in favor of using df::builtin_mats enum entries

Also adjust some comparisons to use more easily understandable values (e.g. <= CREATURE_200 instead of < HIST_FIG_1)

[quietust]

I'm open to leaving some of those constants behind, such as NUM_BUILTIN, since some of them might not be immediately obvious what you should use instead. GROUP_SIZE is something that needs to be removed, though, since while the 3 groups are currently the same size, there's no guarantee that this will continue to be the case in the future.