Make InstanceKlass::implementor return InstanceKlass

To avoid InstanceKlass::cast() calls

  // Access to the implementor of an interface.
  Klass* implementor() const;
  void set_implementor(Klass* k);
  int nof_implementors() const;
  void add_implementor(Klass* k); // k is a new class that implements this interface
  void init_implementor(); // initialize