Move armor to its own location.

After numerous complaints and counterpoints it seems that there's no really **good** reason to prevent armoring as many squares as you like. I also double-checked how armor works, and it basically gets damaged before any other part in the square when a square gets hit.

So, armor now has its own location, meaning it can't stack with other armor parts, but you can armor anything. If you WANT to armor your bed, sure, go ahead; it's unlikely to take damage from anything but explosions, though.

Closes #3897.