[RFC 60] Add binary heaps

[Theodus]

This PR adds binary heaps to the standard collections package. This includes both minimum and maximum priority heaps through the MinHeap and MaxHeap classes.

closes #2948

[SeanTAllen]

@Theodus could you write up something for the release notes for this? short overview of them and why they might be useful?

[Theodus]

Release Notes

RFC 60 adds binary heaps to the Pony standard library. Binary heaps are a useful data structure for processing items based on priority, as in priority queues. A min or max heap may be created through the MinHeap or MaxHeap classes, respectively.

Here is an example of a min-heap containing integers:

use "collections"
use "itertools"

actor Main
  new create(env: Env) =>
    let q = MaxHeap[I64](5) .> append([1; 5; 8; 3; 4])

    // prints `8 5 4 3 1`
    env.out.print(" ".join(
      Iter[USize](Range(0, q.size()))
        .map_stateful[I64]({ref(n)? => q.pop()? })))