core: introduce CheckReturnState constants

At the moment we should have 3 possible options to check return state
during vm context unloading:
	* no check
	* ensure the stack is empty
	* ensure the stack is not empty

It is necessary to distinguish them because new _deploy method shouldn't
left anything on stack. Example: if we use _deploy method before some
ordinary contract method which returns one value. Without these changes
the contract invocation will fail due to 2 elements on stack left after
invocation (the first `null` element is from _deploy, the second element
is return-value from the ordinary contract method).

pkg/vm/context.go

@@ -46,9 +46,22 @@ type Context struct {
 	callFlag smartcontract.CallFlag
 
 	// CheckReturn specifies if amount of return values needs to be checked.
-	CheckReturn bool
+	CheckReturn CheckReturnState
 }
 
+// CheckReturnState represents possible states of stack after opcode.RET was processed.
+type CheckReturnState byte
+
+const (
+	// NoCheck performs no return values check.
+	NoCheck CheckReturnState = 0
+	// EnsureIsEmpty checks that stack is empty and panics if not.
+	EnsureIsEmpty CheckReturnState = 1
+	// EnsureNotEmpty checks that stack contains not more than 1 element and panics if not.
+	// It pushes stackitem.Null on stack in case if there's no elements.
+	EnsureNotEmpty CheckReturnState = 2
+)
+
 var errNoInstParam = errors.New("failed to read instruction parameter")
 
 // NewContext returns a new Context object.