Add SER_READ and SER_WRITE for read/write-dependent statements

Extracted and extended by Pieter Wuille from a comment by Russ
Yanofsky (see
https://github.com/bitcoin/bitcoin/pull/18317#discussion_r398625457).

src/serialize.h

@@ -190,6 +190,8 @@ template<typename X> const X& ReadWriteAsHelper(const X& x) { return x; }
 
 #define READWRITE(...) (::SerReadWriteMany(s, ser_action, __VA_ARGS__))
 #define READWRITEAS(type, obj) (::SerReadWriteMany(s, ser_action, ReadWriteAsHelper<type>(obj)))
+#define SER_READ(obj, code) ::SerRead(s, ser_action, obj, [&](Stream& s, typename std::remove_const<Type>::type& obj) { code; })
+#define SER_WRITE(obj, code) ::SerWrite(s, ser_action, obj, [&](Stream& s, const Type& obj) { code; })
 
 /**
  * Implement three methods for serializable objects. These are actually wrappers over
@@ -1100,6 +1102,28 @@ inline void SerReadWriteMany(Stream& s, CSerActionUnserialize ser_action, Args&&
     ::UnserializeMany(s, args...);
 }
 
+template<typename Stream, typename Type, typename Fn>
+inline void SerRead(Stream& s, CSerActionSerialize ser_action, Type&&, Fn&&)
+{
+}
+
+template<typename Stream, typename Type, typename Fn>
+inline void SerRead(Stream& s, CSerActionUnserialize ser_action, Type&& obj, Fn&& fn)
+{
+    fn(s, std::forward<Type>(obj));
+}
+
+template<typename Stream, typename Type, typename Fn>
+inline void SerWrite(Stream& s, CSerActionSerialize ser_action, Type&& obj, Fn&& fn)
+{
+    fn(s, std::forward<Type>(obj));
+}
+
+template<typename Stream, typename Type, typename Fn>
+inline void SerWrite(Stream& s, CSerActionUnserialize ser_action, Type&&, Fn&&)
+{
+}
+
 template<typename I>
 inline void WriteVarInt(CSizeComputer &s, I n)
 {