Move GVariant serialization into Contacts.Chunk

Rather than building a big if-else block in the Contacts.Io namespace, it's much more interesting to move the GVariant serialization into the Contacts.Chunk objects themselves. That allows us to keep the serialization logic for a specific field in one place and makes sure we don't forget about any properties as they're not part of that big if-else block that checks on property name.

This commit also make sure a lot of the functionality here is now unit tested, to make sure we're not accidentally regressing.