[WIP] [CR] More dynamic food nutrition/enjoyability values

To make future changes to comestibles easier, I created three functions in item.cpp that would hopefully contain all the checks and modifiers for the enjoyability, nutrition and quench of comestibles.
The main use of this is to update the item's tooltip while being examined: If a comestible gives 5 base enjoyability, but a trait the player has adds 10 to that amount, the tooltip would correctly say "Enjoyability: 15" rather than remain at 5. Also works for the enjoyability of books.
Submitting this for review, as even though it currently "just works", I'm sure it could use improvement before I start moving more checks into the functions.