Enable the use of prefix:fragment qnames

- create a lookup table which matches abbreviations to base URIs
- add functions to add entries and perform lookups
- add a function 'expand-qname, which uses the lookup table to transform
  prefix:fragment qnames into RDF URIs
- replace the use of ox:qname with ox:expand-qname wherever feasible
- remove the constants and unexport them
- constants that we weren't actively using are YAGNIed