Add remaining haskell book exercises

Haskell-book/17/Constant.hs

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+module Constant where
+
+newtype Constant a b =
+    Constant { getConstant :: a }
+    deriving (Eq, Ord, Show)
+
+instance Functor (Constant a) where
+    fmap _ (Constant x) = Constant x
+
+instance Monoid a
+  => Applicative (Constant a) where
+      pure x = Constant mempty
+      (Constant x) <*> (Constant y) = Constant $ mappend x y