Python: Add toString information for all classes without such.

Having `toString()` defined to be `none()` is a major headache when debugging,
as `toString`-less results are silently elided. This PR puts dummy `toString`s
in place of the `none()`s.

(I am mostly creating this to see if it impacts our tests and/or the
performance. If not, we may as well merge it.)

python/ql/src/external/CodeDuplication.qll

@@ -93,7 +93,7 @@ class Copy extends @duplication_or_similarity
     }
 
     /** Gets a textual representation of this element. */
-    string toString() { none() }
+    string toString() { result = "Copy" }
 
     /**
      * Gets a block that extends this one, that is, its first token is also