Check stateful annotations

Functions must be annotated as `stateful` in order to
- Update the contract state (using `put`)
- Call `Chain.spend` or other primitive functions that cost tokens
- Call an Oracle or AENS function that requires a signature
- Make a remote call with a non-zero value
- Construct a lambda calling a stateful function

It does not need to be stateful to
- Read the contract state
- Call another contract with value=0, even when the remote function is stateful

test/contracts/simple_storage.aes

@@ -24,5 +24,5 @@ contract SimpleStorage =
 
   function get() : int = state.data
 
-  function set(value : int) =
+  stateful function set(value : int) =
       put(state{data = value})