Fix for Carnivores being able to eat low-calorie veggies

[p4nc4k3z]

Summary

Bugfixes "Fix for Carnivores being able to eat low-calorie veggies"

Purpose of change

Veggies under ~17 calories, including pickles, accidentally slip past the code block which checks whether or not a carnivore is prohibited from eating foods. This PR fixes that.

Describe the solution

Swapping in the method compute_effective_nutrients() for the method nutrition_for() when checking if a carnivore's meal has calories or not. The latter currently calls the former anyway, but introduces the problem when dividing the former result as casting to an int in C++ rounds the quotient down to zero when it's too small.

A comment is also included in nutrition_for() warning developers of this danger.

Describe alternatives you've considered

On Discord, we discussed instead simply giving 1 nutrition whenever nutrition_for() would start with caloric food but round it down to zero. This would still be a reasonable idea, and could safely be done even after this PR is merged, but I decided against it. This would have the following effects:

• Magic food: All super-low-calorie-foods would give slightly more nutrition than they should. (Like one bite of an apple, but, still.)
• Hibernation: Super-low-calorie foods currently skip the hibernation logic entirely, but would instead show messages and contribute (very slightly) to sleepiness.
• Food poisoning: Eating rotten super-low-calorie food currently gives 6m of food poisoning, and would instead give 6-12m.
• Acid/fertilizer: Drinking super-low-calorie acid w/ ACIDPROOF/ACIDBLOOD, or drinking super-low-calorie fertilizer w/ THRESH_PLANT could in theory would give additional calories, but a review of the available foods suggests none are applicable anyway.

Testing

Built from source, and compared a carnivore's ability to eat from a menu of foods before and after the change.

Additional context

Thanks to @GuardianDll, @harakka and @snup. for dev help, and to @GuardianDll for tracking down this bug!