Instead of asking "do you have an entry not in this Vector", let's ask
"do you have an entry not in this HashTable".
(cherry picked from commit 861d46be3e69aeb1ca4a9e75029b4ce653bbcf47)
This allows us to use HeapFunction all of the way down, allowing us
to remove the Handle usage in after_session_callback for
create_new_child_navigable.
(cherry picked from commit fc83653f3c7a650229d2be3eef0f1738a9395db9)
