Mineshaft generation improvements

Given the new depths in world generation, mineshaft should include vertical ones with ladders to spread out the layers of tunnels. Also add some procedurally generated structures as part of the mineshaft such as a wood and stone main entrance at the surface, smelting chamber, walled lava pools for natural illumination, and the carved stone archways and glowing lichen chambers to breakup the mineshaft from just all ending in dead ends.