Fixed bug in policy-based ordered statistics tree

[ongyiumark]

Apparently splay_tree_tag is $O(n)$. We need to us rb_tree_tag to achieve $O(\log n)$.

Sauce: https://codeforces.com/blog/entry/11080#:~:text=splay%20tree%20and%20OV%2Dtree%20using%20linear%2Dtimed%20split%20operation%20that%20prevents%20us%20to%20use%20them.

[leloykun]

seems serious

we're gonna need a test case for this just to be sure

[ongyiumark]

I submitted two identical solutions to this problem. Got TLE with splay_tree_tag.
https://cses.fi/problemset/task/1144/

TLEd on this input:
test_input.txt

AC code:
https://cses.fi/paste/7df91b3af6391324750784/

TLE code:
https://cses.fi/paste/e67c67d2e27cc92a75077e/

[leloykun]

yolo

let's merge this