Improve definition references

[adamgerhant]

Improves the definition reference system by referencing nodes with DefinitionIdentifier instead of strings. This means protonodes use their identifier to reference their definition, instead of an arbitrary string.

Sets the foundation for an improved migration system and linking nodes to their definition when serializing.

For example: instead of

resolve_document_node_type("Star")

you would do

resolve_document_node_type(&DefinitionIdentifier::ProtoNode(graphene_std::vector::generator_nodes::star::IDENTIFIER))

Another example

resolve_document_node_type("Merge")

to

resolve_document_node_type(&DefinitionIdentifer::Network("Merge".to_string()))