Add a functor example

Exercises/exercise-5/FunctorExample.hs

@@ -0,0 +1,10 @@
+-- Generic type
+newtype Data a = Data {value :: a}
+
+-- Implement generic type conversion of `Data`
+instance Functor Data where
+    fmap f (Data val) = Data (f val)
+
+-- >>> value ((*9) <$> (Data { value = 2 }))
+-- 18
+--