feat: struct IntervalSet

[TrueRyoB]

features:
let...

• N be the size of ranges inside the struct
• k be the size of output

1. IntervalSet(ll L, ll R): $O(1)$
2. template<T> void insert(T L, T R) : $O(\log{N})$
3. template<T> T remove(T L, T R): $O(\log{N})$
4. template<T> T covered_length(): $O(\log{N})$
5. template<T> T covered_length(T L, T R): $O(\log{N})$
6. template<T> int size(): $O(\log{N})$
7. template<T> int size(T L, T R): $O(\log{N})$
8. bool intersects(T L, T R): $O(\log{N})$
9. bool contains(T i): $O(\log{N})$
10. vector<pair<T,T>> enumerate(T L, T R): $O(k+ \log{N})$

if I am still feeling okay...

• split
• next_gap
• next_interval
• clear
• ~IntervalSet()