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/init.lua

@@ -4,6 +4,7 @@
 
 modpath = minetest.get_modpath("technic_chests")
 
+technic = technic or {}
 technic.chests = {}
 
 dofile(modpath.."/common.lua")