Added FusedStream impl to Buffered

[jean-airoldie]

Note that the buffered method seem to be missing tests.

[taiki-e]

(Looks like the new FusedStream impl is the same as .buffered().fuse().)

[cramertj]

Mhm, I don't think we should add FusedStream impls to types like this if it means storing an extra boolean, since that's equivalent to wrapping with .fuse().

[jean-airoldie]

Fair enough, I agree with this reasoning. I'll close the PR.

[taiki-e]

As FuturesOrdered is now implements FusedStream (#2205), I think this can be implemented by using self.stream.is_terminated() && self.in_progress_queue.is_terminated().