Merge Compare Flattened Values algorithm into Node Map Generation

The algorithm can be reduced two a single sentence because it will only ever compare value objects or node references, i.e., node objects with only an @id member. Lists, and thus also arrays, are handled separately (step 5 in the Node Map generation algorithm).

Lists are never merged because they are implicit unlabeled blank nodes. If we would merge them, RDF round-tripping would break.

This addresses #212.