PairingHeap is a pure Ruby priority queue implementation using a pairing heap as
the underlying data structure. While a pairing heap is asymptotically less
efficient than the Fibonacci heap, it is usually faster in practice. This makes
it a popular choice for Prim's MST or Dijkstra's algorithm implementations.

PairingHeap is currently being used as the priority queue data structure in RGL.

Also implementation without priority change support is
provided(SimplePairingHeap), while the asymptotical complexity of the methods
stay the same, bookkeeping of elements is not needed making, the constant
smaller.