RGDA1 for RDFlib

RGDA1 (RDF Graph Digest Algorithm 1) combines the traces graph labeling algorithm with the Sayers and Karp RDF graph digest algorithm. This algorithm will produce canonical cryptographically unique identifiers for any RDF graph.
`self.hash_cache` and `self._hash_color` seem to be unused. And you are using a global variable `_hash_cache`. It seems that it is never and is therefore a potential memory leak.