bpo-37029: keep usable_arenas in sorted order without searching (#13612)

This adds a vector of "search fingers" so that usable_arenas can be kept in sorted order (by number of free pools) via constant-time operations instead of linear search.

This should reduce worst-case time for reclaiming a great many objects from O(A**2) to O(A), where A is the number of arenas.  See bpo-37029.