Fix some T4 files for ValueTask overload generation.

Ix.NET/Source/System.Linq.Async/System.Linq.Async.csproj

@@ -41,4 +41,8 @@
     </None>
   </ItemGroup>
 
+  <ItemGroup>
+    <Service Include="{508349b6-6b84-4df5-91f0-309beebad82d}" />
+  </ItemGroup>
+
 </Project>

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/Average.Generated.tt

@@ -71,7 +71,7 @@ foreach (var o in os)
             return source.Select(selector).Average(cancellationToken);
         }
 
-        public static Task<<#=o.res#>> Average<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, Task<<#=o.type#>>> selector)
+        public static Task<<#=o.res#>> Average<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<<#=o.type#>>> selector)
         {
             if (source == null)
                 throw Error.ArgumentNull(nameof(source));
@@ -81,7 +81,7 @@ foreach (var o in os)
             return source.Select(selector).Average(CancellationToken.None);
         }
 
-        public static Task<<#=o.res#>> Average<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, Task<<#=o.type#>>> selector, CancellationToken cancellationToken)
+        public static Task<<#=o.res#>> Average<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<<#=o.type#>>> selector, CancellationToken cancellationToken)
         {
             if (source == null)
                 throw Error.ArgumentNull(nameof(source));

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/MinMax.Generated.tt

@@ -58,7 +58,7 @@ foreach (var m in new[] { "Max", "Min" })
             return <#=m#>Core(source, selector, cancellationToken);
         }
 
-        public static Task<<#=t#>> <#=m#><TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, Task<<#=t#>>> selector)
+        public static Task<<#=t#>> <#=m#><TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<<#=t#>>> selector)
         {
             if (source == null)
                 throw Error.ArgumentNull(nameof(source));
@@ -68,7 +68,7 @@ foreach (var m in new[] { "Max", "Min" })
             return <#=m#>Core(source, selector, CancellationToken.None);
         }
 
-        public static Task<<#=t#>> <#=m#><TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, Task<<#=t#>>> selector, CancellationToken cancellationToken)
+        public static Task<<#=t#>> <#=m#><TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<<#=t#>>> selector, CancellationToken cancellationToken)
         {
             if (source == null)
                 throw Error.ArgumentNull(nameof(source));

Ix.NET/Source/System.Linq.Async/System/Linq/Operators/Sum.Generated.tt

@@ -71,7 +71,7 @@ foreach (var o in os)
             return SumCore(source, selector, cancellationToken);
         }
 
-        public static Task<<#=o.type#>> Sum<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, Task<<#=o.type#>>> selector)
+        public static Task<<#=o.type#>> Sum<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<<#=o.type#>>> selector)
         {
             if (source == null)
                 throw Error.ArgumentNull(nameof(source));
@@ -81,7 +81,7 @@ foreach (var o in os)
             return SumCore(source, selector, CancellationToken.None);
         }
 
-        public static Task<<#=o.type#>> Sum<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, Task<<#=o.type#>>> selector, CancellationToken cancellationToken)
+        public static Task<<#=o.type#>> Sum<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<<#=o.type#>>> selector, CancellationToken cancellationToken)
         {
             if (source == null)
                 throw Error.ArgumentNull(nameof(source));