There have been multiple instances where I had the need to iterate over
type variables, only to discover that the traversal order is arbitrary.
This commit fixes that by adding SortedMapping, which utilizes BTreeMap
internally to guarantee a traversal order. All instances of VarMap are
now refactored to use this to ensure that type variables are iterated in
the order of its variable ID, which should be monotonically incremented
by the unifier.