Single null node representation

[liss-h]

Close #321

Design

• null node's backend id is all zeros. This happens to be the blank node repr, but that doesn't matter (see following points)
• Node::is_{literal,iri,..} now return TriBool. If the node is null: TriBool::Err is returned, otherwise the function reports the type as before.
• {Literal,IRI,BlankNode,Variable}::is_{literal,iri,..} are now deleted, as it does not make sense to call them if you already know the concrete type of the node.

POBR

This is not a POBR breaking change under the assumption that null-nodes are never persisted.

[liss-h]

It might make sense to also modify the comparison functions. I'm pretty sure it will also work like this but it would be cleaner

[liss-h]

I have updated the design to address the concerns (description is also updated), please have another look @nkaralis

[mcb5637]

Looks good to me. Found only some typos.

[nkaralis]

Thank you for the changes. I only have one question