impl Future for stream::{Buffered, BufferUnordered, FuturesOrdered}

[arcnmx]

This enables poll()ing to fill the buffer without producing or consuming the Stream's outputs.

Buffered and FuturesOrdered will also poll their inner futures produced by the stream, since they have an output value buffer available.
BufferUnordered however will only poll the underlying stream, as it cannot produce outputs without consuming them.

Given that the Output of the BufferUnordered impl isn't particularly useful (because it doesn't indicate anything about the futures produced by the stream), maybe it should be type Output = Infallible, and never complete?
For the other two, the completion is more meaningful as it indicates that the stream is empty and that all captured futures have produced outputs (though in the case of Buffered, it will stall while the buffer remains full).

It may also make sense to expose this as a poll_stream() function rather than impl Future (or otherwise a IntoFuture for &Self or something), for the sake of extension trait method resolution.

EDIT: moved to inherent poll_stream() instead, see latest commit. Final name tbd (buffered.poll_all()? poll_buffer()? poll_fill()?)

[taiki-e]

Thanks for the PR. What is the use case for these methods?