Consistently apply override (and not virtual) to overriding virtuals

Three functions were missing `override`, triggering `-Wsuggest-override` on Clang.
Many functions had redundant `virtual ... override`.

hyper_function.h

@@ -29,7 +29,7 @@ struct function_state : function_state_base<R, Args...> {
     function_state_base<R, Args...> *clone() const override {
         return new function_state(*this);
     }
-    virtual funbase* as_funbase() override {
+    funbase* as_funbase() override {
       if(std::is_base_of<funbase, T>::value) return (funbase*) (&t_);
       return nullptr;
       }