Assert that if we generating code for an item, than it is whitelisted

This is a useful debugging tool for us to catch when code generation and
whitelisting have different understandings of the world.

src/codegen/mod.rs

@@ -287,6 +287,7 @@ impl CodeGenerator for Item {
         }
 
         debug!("<Item as CodeGenerator>::codegen: self = {:?}", self);
+        assert!(whitelisted_items.contains(&self.id()));
 
         result.set_seen(self.id());