Improve performance

[agentcooper]

Hi, @andrewrk.

Thanks for this module.

I am using to solve a heavy computation task. It seems that there is a much more performant heap implementation: https://github.com/lemire/FastPriorityQueue.js.

Would you consider a change from Heap to FastPriorityQueue?

[agentcooper]

Also maybe it is time to switch to native ES6 Map and Set ;-)

[theoparis]

I am working on this myself and am going to add it to my utilities package. I hope this is ok with this author.