add Haskell to Rust example

haskell-to-rust/src/Main.hs

@@ -0,0 +1,10 @@
+{-# LANGUAGE ForeignFunctionInterface #-}
+
+import Foreign.C
+
+foreign import ccall "double_input" doubleInput :: CInt -> CInt
+
+main = do
+    let input = 4
+    let output = doubleInput input
+    putStrLn $ show input ++ " * 2 = " ++ show output