Add the Mantle

Adds a magma oceans region to the nether outside the existing nether caverns, which can be reached via tunnels.

Other misc changes:
* chatcomment nether_whereami, a debug aid for knowing which perlin-noise region you are in
* Nether ores no longer obtainable on the ceiling
* Move crafts into crafts.lua
* Add steam to lava cooling, and play bubbling lava upon death by lava
* Add cracked netherbrick
    a decorative node which can only be obtained from dungeons or structures
    I encourage someone to improve or replace the cracked netherbrick texture. For copyright purposes it's currently a derivative work (by me, 2020) from nether_brick.png, which is licensed under CC BY-SA 3.0 by PilzAdam, so it can fall under the "All other media" PilzAdam's credit in readme.md rather than need its own entry.

init.lua

@@ -116,6 +116,7 @@ end
 dofile(nether.path .. "/portal_api.lua")
 dofile(nether.path .. "/nodes.lua")
 dofile(nether.path .. "/tools.lua")
+dofile(nether.path .. "/crafts.lua")
 if nether.NETHER_REALM_ENABLED then
 	if nether.useBiomes then
 		dofile(nether.path .. "/mapgen.lua")
@@ -227,3 +228,18 @@ The expedition parties have found no diamonds or gold, and after an experienced
 
 	})
 end
+
+
+-- Play bubbling lava sounds if player killed by lava
+minetest.register_on_dieplayer(
+	function(player, reason)
+		if reason.node ~= nil and minetest.get_node_group(reason.node, "lava") > 0 or reason.node == "nether:lava_crust" then
+			minetest.sound_play(
+				"nether_lava_bubble",
+				-- this sample was encoded at 3x speed to reduce .ogg file size
+				-- at the expense of higher frequencies, so pitch it down ~3x
+				{to_player = player:get_player_name(), pitch = 0.3, gain = 0.8}
+			)
+		end
+	end
+)