replaced IOMemory store with new implementation based on Python sets
replaced IOMemory store with new implementation based on Python sets

armandobs14/rdflib,armandobs14/rdflib,dbs/rdflib,ssssam/rdflib,RDFLib/rdflib,marma/rdflib,marma/rdflib,dbs/rdflib,dbs/rdflib,yingerj/rdflib,avorio/rdflib,ssssam/rdflib,ssssam/rdflib,marma/rdflib,yingerj/rdflib,ssssam/rdflib,RDFLib/rdflib,avorio/rdflib,marma/rdflib,RDFLib/rdflib,armandobs14/rdflib,avorio/rdflib,dbs/rdflib,armandobs14/rdflib,yingerj/rdflib,yingerj/rdflib,avorio/rdflib,RDFLib/rdflib