+ with the key. If you provide this function, the splay tree
+ will take the ownership of the memory of the splay_tree_key arg
+ of splay_tree_insert. This function is called to release the keys
+ present in the tree when calling splay_tree_delete or splay_tree_remove.
+ If splay_tree_insert is called with a key equal to a key already
+ present in the tree, the old key and old value will be released. */