Merge #12731: Support serialization as another type without casting

818dc74 Support serialization as another type without casting (Pieter Wuille)

Pull request description:

  This adds a `READWRITEAS(type, obj)` macro which serializes `obj` as if it were converted to `const type&` when `const`, and to `type&` when non-`const`. No actual cast is involved, so this only works when this conversion can be done automatically.

  This makes it usable in serialization code that uses a single implementation for both serialization and deserializing, which doesn't know the constness of the object involved.

  This is a redo of #12712, using a slightly different interface.

Tree-SHA512: 262f0257284ff99b5ffaec9b997c194e221522ba35c3ac8eaa9bb344449d7ea0a314de254dc77449fa7aaa600f8cd9a24da65aade8c1ec6aa80c6e9a7bba5ca7

src/addrman.h

@@ -59,7 +59,7 @@ public:
 
     template <typename Stream, typename Operation>
     inline void SerializationOp(Stream& s, Operation ser_action) {
-        READWRITE(*static_cast<CAddress*>(this));
+        READWRITEAS(CAddress, *this);
         READWRITE(source);
         READWRITE(nLastSuccess);
         READWRITE(nAttempts);