Ignore TODO for now
Add quoting tests, and slightly better "rendering"
Make unification algorithm a bit simpler

* Also uses a `Subst` instance for stacktypes over types, instead of separate
  code in the unifier.
* Using `++` was supremely slow for terms like `[dup] i` - switching to `union`
  fixed this. Obviously these should be changes to sets at some point.
Cleanup unification functions a bit more

* Just noodling at this point, but this looks much cleaner.