Add remaining haskell book exercises

Haskell-book/15/semigroup/app/Main.hs

@@ -0,0 +1,18 @@
+module Main where
+
+import Data.Monoid
+import Mem
+
+f' :: Num a => Mem a String
+f' = Mem $ \s -> ("hi", s + 1)
+
+main :: IO ()
+main = do
+    let rmzero = runMem mempty 0
+        rmleft = runMem (f' <> mempty) 0
+        rmright = runMem (mempty <> f') 0
+    print $ (rmleft :: (String, Int))
+    print $ (rmright :: (String, Int))
+    print $ (rmzero :: (String, Int))
+    print $ rmleft == runMem f' 0
+    print $ rmright == runMem f' 0