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
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