Make ToXml and ToJson work via the pocos, not PocoElementNode

[ewoutkramer]

ToXml(Base) and ToJson(Base) still work via PocoElementNode, but should work via the more native route. This is in principle independent of the new direct implementation of ITypedElement, but in line with making the new serializers work on IScopedNode. It also takes these two extension methods logicalky out of the domain of typedelement, and into the poco domain.