1. CLEANUP: Unified reading/asserting/retracting literals.
2. ENHANCED: Lterals can be attributed to literals using blank nodes.
3. FIXED: term<->bnode map would always set a new bnode, but sometimes we only want to read and fail if there is no such map.
4. CLEANUP: Simplified RDFS label assertion/retraction/reading by building it on to of literal support.