New message pack spec

In v0.3.2, MsgPack-CLI support unpacking new types. ** str8 will be deserialized as 'raw bytes'. ** bin8, bin16, bin32 will be deserialized as byte array. It will not be deserialized as string. ** fixext* and 'ext*' will be deserialized as single MessagePackObject which contains 2 elements array where 1st element is type identifier(Byte) and 2nd element is opaque binary(Byte[]). ** Effectively, it means that above types will not cause MessageTypeException on serializer.
In v0.4.0, MsgPack-CLI support packing new types. ** str8 will be used on 32-255 bytes utf8 string. ** bin8, bin16, bin32 will be used for bytes array and strings which are not encoded in utf8. ** You can implement custom serializer using brand-new extension type with new extension packing API on Packer. ** SerializationContext.CompatibilityOptions will expose a new property which controls use new types in packing. ** New wiki page which offers guidance to implemnt custom serializers which utilize extension type.

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