Writer should de-dup BNode identifiers that are shared across different nodes

[gkellogg]

This can happen either due to intentionally adding different nodes with the same identifier, or reading multiple documents that happen to use the same identifiers into a repository.

Because Writer does not operate on repositories, but using #write_statement, this can't be done at the repo level. Instead, a writer must implement a memory similar to the way readers do to map BNodes to their ids, and create a new (or modified) identifier if it's already been assigned.