#8 Proper graph canonization and hashing

Implement proper graph canonization, that takes blank nodes into account. This was partially implemented in https://github.com/scossu/rdf_canon/blob/master/rdf_hash/rdf_hash.py based on a white paper (see project README), and requires 1. Porting to `lsup_rdf` 2. Resolving further blank node related issues (see README details). Once canonization is achieved, it should be used as the base for `LSUP_graph_hash()` for identification and isomorphism tests.