copilot: Update example to demonstrate struct update support. Refs #524.

This commit updates the `what4-structs` example in `copilot` (which depends on
`copilot-theorem`) in order to demonstrate that `copilot-theorem` supports
reasoning about a basic theorem involving a struct field update.

Author: RyanGlScott

copilot/examples/what4/Structs.hs

@@ -64,6 +64,10 @@ spec = do
   void $ prop "Example 2" $ forAll $
     (((battery#other) .!! 2) .!! 3) == (((battery#other) .!! 2) .!! 4)
 
+  -- Update a struct field, then check it for equality.
+  void $ prop "Example 3" $ forAll $
+    ((battery ## temp =$ (+1))#temp == (battery#temp + 1))
+
 main :: IO ()
 main = do
   spec' <- reify spec