[Yaml-core] Syck's special unkown type classes

[TRANS <tra...@gm...>]

In Syck when an unknown  type is encountered, it creates a special
obect, eg.

#<YAML::PrivateType:0xb7b672a4 @value={"a"=>1, "b"=>2}, @type_id="foo">

One can of course get to the underlying data via #value, but it would
be better if the class was a subclass of the actually YAML type. In
other words, for the above example, a YAML::PrivateMap < Hash. That
way they can be still be treated normally. As it is either one *must*
decalre add_private_type or make special account when traversing the
data, eg. 'case node when YAML::PrivateType'.

Thanks,
T.