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/fundme.aes

@@ -15,7 +15,7 @@ contract FundMe =
   private function require(b : bool, err : string) =
     if(!b) abort(err)
 
-  private function spend(args : spend_args) =
+  private stateful function spend(args : spend_args) =
     Chain.spend(args.recipient, args.amount)
 
   public function init(beneficiary, deadline, goal) : state =