gen-class should allow exposes-methods to expose protected final methods

Currently, there is no way in Clojure to call a protected final method of a superclass. While this may be an acceptable limitation for proxy, gen-class should provide that ability. Otherwise, one is now forced to create a dummy subclass in Java for the sole purpose of widening the visibility of such methods.

My patch makes it so that protected final methods may be exposed via the :exposes-methods mechanism. It is still impossible to override them.