This patch adds a new pragma so that users can specify COMPLETE sets of
ConLikes in order to sate the pattern match checker.
Things which need to be resolved:
- Write a specification
- Documentation in the user guide
- Beef up typechecking to check the set of patterns in consistent
- Persist constructor sets between modules
- Allow COMPLETE sigs to mention imported ids.
- Resolve what should become of the copied ListT code.
- Move test files into the pmcheck directory.
- Add annotations to the parser