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.
Thanks, your first suggestion is what I intended. The idea is that the hash_cache is actually re-used by all colors within a canonicalization, obviating the need to re-hash existing colors. This code review has been very helpful!