[Tolk] Implement AST: intermediate representation of tolk files

Now, the whole .tolk file can be loaded as AST tree and
then converted to Expr/Op.
This gives a great ability to implement AST transformations.
In the future, more and more code analysis will be moved out of legacy to AST-level.

tolk/unify-types.cpp

@@ -268,8 +268,6 @@ std::ostream& TypeExpr::print(std::ostream& os, int lex_level) const {
           return os << "cont";
         case _Tuple:
           return os << "tuple";
-        case _Type:
-          return os << "type";
         default:
           return os << "atomic-type-" << value;
       }