Fix compiler warning about no await operators

[rmunn]

Fix #1042.

This PR fixes the compiler warning "This async method lacks 'await' operators and will run synchronously" by changing the foreach (var foo in SomeEnumerable) yield return new Foo { ... } code to use LINQ instead, i.e. SomeEnumerable.ToAsyncEnumerable().Select(foo => new Foo { ... }.

[rmunn]

There's one question I could not make up my mind about: after running .OrderBy on the sync enumerable, is it better to do .ToAsyncEnumerable().Select(item => new Foo { ... })? Or should the .Select be done on the sync enumrable, and then the final result converted to an async enumerable, i.e. .Select(item => new Foo { ... }).ToAsyncEnumerable()? Or (as I think is correct) does it not actually matter at all?

[hahn-kev]

I don't think it matters