* ref: KeyPriorityQueue in separate file #1298
* feat: add tests for KeyPriorityQueue #1298
* fix: _shiftDown refactored and corrected #1298
* fix: use KeyPriorityQueue in PrimMST #1298
* feat: add test for PrimMST #1298
* fix: format files #1298
* fix: minor coding style changes
* fix: use map for keys and priorities #1298