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/basic_auth.aes

@@ -4,7 +4,7 @@ contract BasicAuth =
 
   function init() = { nonce = 1, owner = Call.caller }
 
-  function authorize(n : int, s : signature) : bool =
+  stateful function authorize(n : int, s : signature) : bool =
     require(n >= state.nonce, "Nonce too low")
     require(n =< state.nonce, "Nonce too high")
     put(state{ nonce = n + 1 })