Add closures and fix lets in monomorphizer

sample-programs/working/map.crf

@@ -0,0 +1,16 @@
+
+data List a where 
+    Nil  : List a
+    Cons : a -> List a -> List a
+
+map : (a -> b) -> List a -> List b
+map f xs = case xs of
+    Nil       => Nil
+    Cons x xs => Cons (f x) (map f xs)
+
+sum : List Int -> Int
+sum xs = case xs of
+    Nil       => 0
+    Cons x xs => x + (sum xs)
+
+main = let y = 10 in sum (map (\x. x + y) (Cons 2 (Cons 4 Nil)))