Allow eating edible items

Add menu support to the prompt system, and an "Eat" command that prompts
for an item to eat and eats the item the character specifies, restoring
an amount of hitpoints configurable via the item raw type.

test/Xanthous/UtilSpec.hs

@@ -0,0 +1,24 @@
+module Xanthous.UtilSpec (main, test) where
+
+import Test.Prelude
+import Xanthous.Util
+
+main :: IO ()
+main = defaultMain test
+
+test :: TestTree
+test = testGroup "Xanthous.Util"
+  [ testGroup "smallestNotIn"
+    [ testCase "examples" $ do
+        smallestNotIn [7 :: Word, 3, 7] @?= 0
+        smallestNotIn [7 :: Word, 0, 1, 3, 7] @?= 2
+    , testProperty "returns an element not in the list" $ \(xs :: [Word]) ->
+        smallestNotIn xs `notElem` xs
+    , testProperty "pred return is in the list" $ \(xs :: [Word]) ->
+        let res = smallestNotIn xs
+        in res /= 0 ==> pred res `elem` xs
+    , testProperty "ignores order" $ \(xs :: [Word]) ->
+        forAll (shuffle xs) $ \shuffledXs ->
+          smallestNotIn xs === smallestNotIn shuffledXs
+    ]
+  ]