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.
`_TripleCanonicalizer` takes a has function that behaves like the ones from `hashlib`. But `_sha256_hash` is different. Wouldn't it be correct to ommit the second argument, such that the default (`haslib.sha256`) is used? If yes, you can even remove the method `_sha256_hash` altogether.

Also, you should update the comment above, as its no longer "MD5 checksums".