Derive OrderedEnumerable from Iterator (#98874)

* Move OrderedEnumerable to be nested in Enumerable

* Derive OrderedEnumerable from Iterator

The OrderedEnumerable implementation today doesn't derive from Iterator, and it has a GetEnumerator method implemented with yield. That means the optimization that allows the enumerable to be reused as the enumerator doesn't kick in, and we end up allocating a separate enumerator object in order to iterate through an Order{By} enumerable.

Author: stephentoub

src/libraries/System.Linq/src/System/Linq/Distinct.SpeedOpt.cs

@@ -9,7 +9,7 @@ public static partial class Enumerable
     {
         private sealed partial class DistinctIterator<TSource> : IIListProvider<TSource>
         {
-            public TSource[] ToArray() => Enumerable.HashSetToArray(new HashSet<TSource>(_source, _comparer));
+            public TSource[] ToArray() => HashSetToArray(new HashSet<TSource>(_source, _comparer));
 
             public List<TSource> ToList() => new List<TSource>(new HashSet<TSource>(_source, _comparer));