Add union type support

[nsensfel]

(declare_union #item
   (weapon #weapon)
   (armor #armor)
   (consumable #consumable)
   nothing
)

   (lambda ((#item i))
      (switch i
         ((weapon w) (eval is_sword w))
         ((armor a) (false))
         ((consumable a) (false))
         (nothing (false))
      )
   )