Add ImmutableArray extension methods for ordering

[DustinCampbell]

This change introduces four sets of extension methods with overloads:

• OrderAsArray(...)
• OrderDescendingAsArray(...)
• OrderByAsArray(...)
• OrderByDescendingAsArray(...)

Each of these operates on an ImmutableArray<T> and returns an ImmutableArray<T>.

[davidwengier]

Other than general YAGNI, I have nothing against this PR... but didn't I just see a comment on another PR that mentioned there was a Sort method on ImmutableArray already?

It's odd to me that these exist if there is a Sort method, and/or that these don't use that Sort method.

What am I missing?

[DustinCampbell]

Other than general YAGNI, I have nothing against this PR... but didn't I just see a comment on another PR that mentioned there was a Sort method on ImmutableArray already?

The primary reason is that OrderBy is really useful, especially when you want to sort by some key. ImmutableArray's Sort would require an IComparer<T> or Comparison<T> to sort by a key, so it can often be inconvenient. Also, with ImmutableArray flowing around a lot more through type-inferred variables. It's just too easy to use the LINQ OrderBy without noticing that you've boxed the ImmutableArray and transforming to Sort can change the calling code significantly. So, I understand the feeling that this is YAGNI, but I think #10598 is a good example of why it's useful.

It's odd to me that these exist if there is a Sort method, and/or that these don't use that Sort method.

OrderBy could use the Sort method, but that would mean creating a new IComparer<T> each call to wrap the keyComparer. Then, the keyComparer would need to be called many, many times on the x and y values in each Compare call. Since the runtime provides an optimized method for setting an array of keys and items, I opted to use that and only call keyComparer once for each item. For Order, I think using Sort is a fair point, but it would be problematic for OrderDescending.

[DustinCampbell]

@davidwengier / @ryzngard / @333fred - Feel free to take another look. The implementation has changed pretty significantly.