Don't overwrite registered crafts

The crafts_table was being initialised to contain only core-registered
crafts, but this would overwrite any non-core craft types that had
already been registered via unified_inventory.register_craft().  This was
especially likely because the crafts_table initialisation runs on a delay,
following all normal initialisation.  Instead, feed the core-registered
crafts into unified_inventory.register_craft().