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

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.

Start by reviewing the heap update methods (`update`, `increase`, and `decrease`), `ordered_begin()`, and `s_handle_from_iterator` to understand the existing iterator and handle APIs. Resolve which proposed top-element or ordered-iterator access is wanted, then verify that the chosen API supports finding and updating k-maximum or k-minimum values as described.

Written by the indexing model from the issue text.