Reduce dependencies of technic_chests

technic_chests was depending on the technic mod, for the top-level
"technic" table and the technic.swap_node function.  Resolve that by
sharing the top-level table and inlining the one use of the function.
It was also depending on technic_worldgen, for the definitions of
cast iron and wrought iron.  Make the use of cast iron conditional on
technic_worldgen, falling back to default "steel".  Change the use of
wrought iron to directly use default "steel", to which it is aliased
anyway.

technic_chests/register.lua

@@ -49,7 +49,9 @@ end
 local function check_color_buttons(pos, meta, chest_name, fields)
 	for i = 1, 16 do
 		if fields["color_button"..i] then
-			technic.swap_node(pos, chest_name..colorid_to_postfix(i))
+			local node = minetest.get_node(pos)
+			node.name = chest_name..colorid_to_postfix(i)
+			minetest.swap_node(pos, node)
 			meta:set_string("color", i)
 			return
 		end