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/counter.aes

@@ -5,5 +5,5 @@ contract Counter =
 
   function init(val) = { value = val }
   function get()     = state.value
-  function tick()    = put(state{ value = state.value + 1 })
+  stateful function tick()    = put(state{ value = state.value + 1 })