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/factorial.aes

@@ -9,7 +9,7 @@ contract Factorial =
 
   function init(worker) = {worker = worker}
 
-  function set_worker(worker) = put(state{worker = worker})
+  stateful function set_worker(worker) = put(state{worker = worker})
 
   function fac(x : int) : int =
     if(x == 0) 1