Added async singleton collection

Author: sakno

src/DotNext.Tests/Collections/Generic/AsyncEnumerableTests.cs

@@ -97,4 +97,11 @@ public static async Task SkipNullsTestAsync()
         True(Array.Exists(array, "a".Equals));
         True(Array.Exists(array, "b".Equals));
     }
+
+    [Fact]
+    public static void Singleton()
+    {
+        var enumerable = AsyncEnumerable.Singleton(42);
+        Equal(42, Single(enumerable));
+    }
 }

src/DotNext/Collections/Generic/AsyncEnumerable.cs

@@ -219,4 +219,13 @@ public static IAsyncEnumerable<T> Throw<T>(Exception e)
 
         return new ThrowingEnumerator<T>(e);
     }
+
+    /// <summary>
+    /// Constructs read-only sequence with a single item in it.
+    /// </summary>
+    /// <param name="item">An item to be placed into list.</param>
+    /// <typeparam name="T">Type of list items.</typeparam>
+    /// <returns>Read-only list containing single item.</returns>
+    public static IAsyncEnumerable<T> Singleton<T>(T item)
+        => new Specialized.SingletonList<T> { Item = item };
 }

src/DotNext/Collections/Specialized/SingletonList.cs

@@ -12,7 +12,7 @@ namespace DotNext.Collections.Specialized;
 /// </summary>
 /// <typeparam name="T">The type of the element in the list.</typeparam>
 [StructLayout(LayoutKind.Auto)]
-public struct SingletonList<T> : IReadOnlyList<T>, IList<T>, ITuple, IReadOnlySet<T>
+public struct SingletonList<T> : IReadOnlyList<T>, IList<T>, ITuple, IReadOnlySet<T>, IAsyncEnumerable<T>
 {
     /// <summary>
     /// Represents an enumerator over the collection containing a single element.
@@ -145,6 +145,10 @@ readonly IEnumerator<T> IEnumerable<T>.GetEnumerator()
     readonly IEnumerator IEnumerable.GetEnumerator()
         => GetEnumerator().ToClassicEnumerator<Enumerator, T>();
 
+    /// <inheritdoc />
+    readonly IAsyncEnumerator<T> IAsyncEnumerable<T>.GetAsyncEnumerator(CancellationToken token)
+        => GetEnumerator().ToAsyncEnumerator<Enumerator, T>(token);
+
     /// <summary>
     /// Converts a value to the read-only list.
     /// </summary>