IAsyncGrouping is inconvinient

In `System.Linq.Async`, GroupBy returns `IAsyncGrouping` but internally it is `IGrouping` and cast it.

> `(IAsyncGrouping)_enumerator.Current`

IAsyncGrouping is very inconvenient.
For example, all those who follow can't use the normal methods and have to use ***Await.

```csharp
AsyncEnumerable.Range(1, 10)
.GroupBy(x => x)
.SelectAwait(async x => await x.CountAsync()) // can not use standard .Select operator.
```

This is both an inconvenience and a poor performer.

As a suggestion, why don't you get rid of IAsyncGrouping and return IGrouping?
Since ToLookup implements the standard ILookup,IGrouping, I don't think it's strange if the paired GroupBy is IGrouping.

Start by reviewing System.Linq.Async's GroupBy result and the IAsyncGrouping cast described in the issue, then compare that behavior with ToLookup's IGrouping implementation. Check how standard Select and CountAsync usage would be affected. Done means the API direction is resolved and the chosen behavior is covered by tests.

Written by the indexing model from the issue text.