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/dutch_auction.aes

@@ -10,7 +10,7 @@ contract DutchAuction =
                    sold         : bool }
 
   // Add to work around current lack of predefined functions
-  private function spend(to, amount) =
+  private stateful function spend(to, amount) =
     let total = Contract.balance
     Chain.spend(to, amount)
     total - amount