add diffing for other

In the weak correctness (default mode), output signals must be
different, so diffing the output signals are informative enough.

But in the strong corectness (when --strong is given), output signals
could be identical. It would be more helpful to diff the internal
signals to see what are different.

This commit adds diffing for the internal variables.

picus.rkt

@@ -322,8 +322,8 @@
    (format-cex "inputs" in)
    (format-cex "first possible outputs" out1 #:diff out2)
    (format-cex "second possible outputs" out2 #:diff out1)
-   (format-cex "first internal variables" other1)
-   (format-cex "second internal variables" other2)
+   (format-cex "first internal variables" other1 #:diff other2)
+   (format-cex "second internal variables" other2 #:diff other1)
 
    (when arg-wtns
      (parameterize ([current-directory arg-wtns])