StreamExt::forward should be TryStreamExt::forward

[benesch]

The current implementation of StreamExt::forward should actually be in TryStreamExt as try_forward, I think, because it requires a fallible stream. StreamExt should then provide a forward method that does not require the sink to produce Results.

In other words the fix that merged in #1946 doesn't much sense to me. The code that was there prior to #1946 should be resurrected as StreamExt::forward, as the code that is there now should be TryStreamExt::try_forward.

Or is there some edge case that I'm missing? Happy to submit a PR if what I described above seems reasonable.

/cc @cramertj