Proposal: heap enhancements

[kmsquire]

There are a couple of enhancements that may make heaps in this package more useful:

1. Allow the user to specify an Ordering (as is used by the Sort machinery in Base).
2. Make key-value pair storage possible/easy

The second proposal could be made possible by a specific Ordering, but would be more efficient if it were coded for in the data structure.

It may make sense to code these as separate data structures (I haven't looked at the code), implementing the AbstractHeap interface if that makes sense.

[wsliang]

What's the relationship between the heaps in this package and the PriorityQueue / heap* operations in Base.Collections? They seem to have more or less the same functionality.

[oxinabox]

The first is basically covered by / overlaps with #243