Integrate copper items into loot pools

Copper items have not been integrated into generated loot pools (chest loot). The only copper items that appear in generated loot pools are copper horse armor, copper nautilus shell, and copper spear.

Copper ingots, copper nuggets, copper tools, and copper armor should appear in generated loot pools. Gold, iron, and diamond items are included in generated loot pools, this should be the same for copper items.

Villages, ocean ruins, mineshafts, dungeons (monster room), and trial chambers would be reasonable locations to find copper items. 

Copper tools/armor should be an alternative offer to the stone tool/chainmail trade offers of villagers. Or trading should be expanded to include both at different levels.