0.3.0: Rename comprehension check to for-loop check and update related code

- Rename `comprehension_one_prefix.py` to `for_loop_one_prefix.py`
- Update class name from `COP015ComprehensionOnePrefixCheck` to
  `COP015ForLoopOnePrefixCheck`
- Change violation code description from "Comprehension variables" to
  "For-loop variables"
- Add tests for regular for-loop cases
- Rename loop variables in several files for clarity
- Remove unused import `MIN_NAME_LENGTH`