Enhance usability for finding k-maximum (minimum) values in set

[rcollyer]

My chief use case for a heap is finding the k-maximum (minimum) members of a set, but the update methods (update, increase, and decrease) cannot be easily used for this because they take a handle_type which cannot be easily acquired for the top element. Using heap::ordered_begin() does not work because there is no direct route from an ordered_iterator to a handle like in s_handle_from_iterator.

I propose

1. overloading s_handle_from_iterator to accept ordered_iterator,
2. providing access to the top element as a handle, e.g. handle_type top_element_handle();, and
3. possibly adding update methods that act directly on the top element, e.g. void update_top(const_reference), etc.