Assign consistent bnode identifiers in N3 formulas

[eyusupov]

Summary of changes

Blank nodes are assigned inconsistent identifiers when they are within different quoted graphs. I could not find a normative section in the spec that would define the scope of blank nodes, but section 3.6.1 says that we "Then, we use this identifier within our N3 graph", where "N3 document represents an N3 graph".

Checklist

• [ ] Checked that there aren't other open pull requests for the same change.
• [ ] Checked that all tests and type checking passes.
• If the change has a potential impact on users of this project:
  • [x] Added or updated tests that fail without the change.
  • [ ] Updated relevant documentation to avoid inaccuracies.
  • [ ] Considered adding additional documentation.
• [ ] Considered granting push permissions to the PR branch, so maintainers can fix minor issues and keep your PR up to date.

[eyusupov]

Note: interestingly, QuotedGraph documentation mentions that it's intended to be completely disjoint from other graphs https://github.com/RDFLib/rdflib/blob/main/rdflib/graph.py#L33-L49, which contradicts my understanding of the situation

[eyusupov]

Looks like this is expected behavior at the moment: https://w3c.github.io/N3/spec/semantics.html#example3