Using configured enumerator rather than interface.

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/OrderedAsyncEnumerable.cs

@@ -265,7 +265,7 @@ namespace System.Linq
         public async ValueTask<Maybe<TElement>> TryGetFirstAsync(CancellationToken cancellationToken)
         {
 #if CSHARP8
-            await using (IAsyncEnumerator<TElement> e = _source.GetAsyncEnumerator(cancellationToken).ConfigureAwait(false))
+            await using (var e = _source.GetAsyncEnumerator(cancellationToken).ConfigureAwait(false))
             {
                 if (!await e.MoveNextAsync())
                 {
@@ -328,7 +328,7 @@ namespace System.Linq
         public async ValueTask<Maybe<TElement>> TryGetLastAsync(CancellationToken cancellationToken)
         {
 #if CSHARP8
-            await using (IAsyncEnumerator<TElement> e = _source.GetAsyncEnumerator(cancellationToken).ConfigureAwait(false))
+            await using (var e = _source.GetAsyncEnumerator(cancellationToken).ConfigureAwait(false))
             {
                 if (!await e.MoveNextAsync())
                 {