* 1.0
- Initial release.
* 2.0
- Rewritten to implement a priority queue.
- Adapted to course 24-25.
* 2.0.1
- Fixed wrong names shit_up an shit_down.
* 2.0.2
- Fix typo in test name for heapify operation in README.
* 2.0.3
- Fix. include <cassert> in priority_queue_imp.hpp.
* 2.1
- Remove destructors applying zero-rule.