While it is still technically possible to test stage 0, it is not part of any
of the main testing rules and maintaining xfail-stage0 is a chore. Nobody
should worry about how tests fare in stage0.
- Remove redundant uses of 'resolve' and 'referent' in semant.
- Use defn, defn_id, lval, lval_base more consistently.
- Make associated query functions more consistent.
- Closes#127.