rewrite enum_morphisms & find_morphism_path

- introduce `MorphismInstantiation` which instantiates a
  morphism-template using a type-substitution and a halo type.
- find_morphism_path returns list of `MorphismInstatiation`.

src/test/unification.rs

@@ -132,6 +132,7 @@ fn test_unification() {
 #[test]
 fn test_subtype_unification() {
     let mut dict = BimapTypeDict::new();
+
     dict.add_varname(String::from("T"));
     dict.add_varname(String::from("U"));
     dict.add_varname(String::from("V"));

src/unification.rs

@@ -42,7 +42,6 @@ impl UnificationProblem {
         match (lhs.clone(), rhs.clone()) {
             (TypeTerm::TypeID(TypeID::Var(varid)), t) |
             (t, TypeTerm::TypeID(TypeID::Var(varid))) => {
-
                 if ! t.contains_var( varid ) {
                     self.σ.insert(TypeID::Var(varid), t.clone());
                     self.reapply_subst();
@@ -253,6 +252,7 @@ impl UnificationProblem {
         let mut halo_type = TypeTerm::Ladder(halo_rungs);
         halo_type = halo_type.normalize();
         halo_type = halo_type.apply_substitution(&|k| self.σ.get(k).cloned()).clone();
+
         Ok((halo_type.param_normalize(), self.σ))
     }
 }