Make EntitySet<T> implement IAsyncEnumerable<T> so that we can use ToListAsync() on EntitySet<T>

[axmty]

#394

In previous version of DataObjects, we could use AsAsync() on both IQueryable<T> and EntitySet<T>. Now that AsAsync() has been removed, we expect ToListAsync() to work for both IQueryable<T> and EntitySet<T> types, but there is a runtime error since EntitySet<T> does not implement IAsyncEnumerable<T>.