Add check of class redeclaration and relation between superclasses (cannot occur in a class definition). Add a mle test file for superclasses relation.
Add type-checking and well-formedness checking of type classes:

* Check uniqueness of class identifier.
* Check types of method declarations.
* Check single declaration of method.
* Check that superclasses are independent (it doesn't exist i,j such that Ki < Kj).x

* Add test for superclasses independence.
Add two bad tests for typeclass checks.