[index] Support objects of type convertible to value_type in rtree::count().
[test][index] Add test for rtree::count() taking objects of type convertible to value_type.
[index] Refactor count rtree visitor.

Remove specialization of count visitor for value_type, don't duplicate the same algorithm in 2 places.
Move Value/Indexable-aware parts into smaller helper struct and specialize it.