/claim #682

architecture tldr

• old: MessagePackEncoder/MessagePackDecoder classes that wrap msgpack-core.
• new: MessagePackBinaryCodec[A] with auto-deriver, thread-local buffer pool, etc. The code architecture itself mirrored how the schema-toon done

diff highlight:

Added Either[A, B] support to the core schema library, which is required for MessagePack Either serialization (encoded as a single-entry map with "left" or "right" keys).