Motivation: A server sends data packages that consist of multiple
serialized capnproto messages of different structures. Every message is
guaranteed to have the same first field, which works as a message header
containing information about the message structure type.
The scheme comprises the `UnknownMessage` structure that allows parsing
the header only.
Solution: provide a public interface that iterates buffer with
AnyPointer readers to cast a message to `UnknownMessage` first and then
to a specific structure type.
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