Use Base.Ordering with heaps

[jlumpe]

Note: this is basically a duplicate of #547, which still seems active. I wrote all this before finding that one, so I'm submitting it anyways just in case any of it is still useful.

Changes made:

• comp::Comp arguments and fields replaced with order::Base.Ordering everywhere
  • Heaps now support arbitrary orderings
  • LessThan and GreaterThan singleton types and compare function replaced with Base.Forward, Base.Reverse, and Base.Order.lt.
• Heap constructors now take an Ordering as a constructor argument rather than a type parameter, because in general we can't get an instance from just the type (exception follows)
• Min/max type aliases are now const BinaryMinHeap{T} = BinaryHeap{T, typeof(Forward)} and const BinaryMaxHeap{T} = BinaryHeap{T, typeof(Reverse)}, and similarly for mutable version
  • These required adding additional constructors to BinaryHeap/MutableBinaryHeap for these two special cases where the order is a singleton: BinaryHeap{T, typeof(Forward)}(...) = BinaryHeap{T}(Forward, ...) etc
• BinaryHeap, MutableBinaryHeap, nsmallest, and nlargest now have methods with the lt, by, and rev keyword arguments like in sort, which is more user-friendly than requiring an Ordering instance.

This introduces similar breaking changes as in #547 in that isless is now used for comparison by default instead of <, which may yield different results where NaNs are involved (although, I would say that the previous behavior was undefined).
I haven't updated or run the benchmark script yet so I don't know if there are any performance regressions.

[oxinabox]

Thanks for the PR.
As you correctly identified this is redundant now that we have merged #547

[jlumpe]

I think the one thing not included is the ability to use custom orderings which are not singletons. I can create a new PR for that.

[jlumpe]

@oxinabox new PR is here: #673