Implement handling of 'Release' messages.

c++/src/kj/one-of.h

@@ -52,6 +52,9 @@ public:
   OneOf& operator=(const OneOf& other) { if (tag != 0) destroy(); copyFrom(other); return *this; }
   OneOf& operator=(OneOf&& other) { if (tag != 0) destroy(); moveFrom(other); return *this; }
 
+  inline bool operator==(decltype(nullptr)) const { return tag == 0; }
+  inline bool operator!=(decltype(nullptr)) const { return tag != 0; }
+
   template <typename T>
   bool is() const {
     return tag == typeIndex<T>();