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/remote_call.aes

@@ -11,7 +11,7 @@ contract Remote3 =
 
 contract RemoteCall =
 
-    function call(r : Remote1, x : int) : int =
+    stateful function call(r : Remote1, x : int) : int =
         r.main(gas = 10000, value = 10, x)
 
     function staged_call(r1 : Remote1, r2 : Remote2, x : int) =