Use `left` and `right` constants from trees vocabulary

trees/cb/cb.factor

@@ -35,10 +35,8 @@ TUPLE: cb-node { byte# integer } { bits integer } left right ;
 : <cb-node> ( byte# bits -- node )
     cb-node new-node ;
 
-! -1 = left
-! 1 = right
 : key-side ( bits byte -- side )
-    bitor 1 + -8 shift 0 = -1 1 ? ;
+    bitor 1 + -8 shift 0 = left right ? ;
 
 ! Produce a byte with all bits set except the msb from bits*.
 ! See MAGIC Algorithms for rationale.