Overhauled Loot

Changed our loot generation. Instead of relying on Minecraft's built-in
loot categories, we now have our own static loot category. There are two
types of chests now. Regular chests contain randomly-picked coal, iron,
gold, diamonds, emeralds, quartz, enchanted books, and golden apples
(very rare). We also have a function for filling out "grave chests",
which occur in 1 out of every 7 chests. Those contain rotten flesh,
bones, and some armor pieces and equipment that are assigned random
damage values and occasionally also get level 1 enchantments. Small
changes were made to various files to incorporate this update.