Unify useable/eatable/spell code

first commit [317]:

• moves food over to effects.c
• moves item nutrition info to lib/object.txt

issues:

• doesn't print any use information on the "examine" screen

[318] moves scrolls to effects.c

• [319] moved rings across and added description code
• [320] moved artifacts across and further improved description code

current issues:

• items are described to you regardless of ID status.
• rods will not use charge

to bear in mind when moving rods across: make sure they don't give type away by not always prompting for a direction

[321] fixes things that requiring aiming before use, moves the effect code from taking an object to taking an effect type, adds proper display of artifact activation messages, and moves potions to the new code.

Also unifies some bad effects between mushrooms and potions; change duration dice of some effects to more aesethically pleasing.

new issues:

• descriptions need proof-reading
• some effects may still be using "break;" rather than "return TRUE;"

report: "Attempting to read the scroll of trap detection on the ground in my square gives the message "Oops. That object cannot be activated." Other scrolls seem OK. I don't know if all trap detection scrolls ause this issue or only this particular one, since I normally use the spell."

[325] finishes off objects for now, including all the issues noted above. Spells yet to be done.

assigning to no-one, since I'm not working on this for a while