Added ChunksLazy to Stream

[fisherdarling]

This implements a "lazy" iterator over the chunks of a stream. The only difference between ChunksLazy and Chunks is that ChunksLazy yields the local buffer if the underlying stream returns Poll::Pending, even if there are no elements in the buffer.

This closes #1919

[mzabaluev]

"Lazy" may be a misleading term; we have future::lazy which means something completely different.
I suggest renaming it to chunks_eager/ChunksEager, as in eager to yield the buffer as soon as no more items are immediately forthcoming.

[mzabaluev]

even if there are no elements in the buffer.

I don't think there is value in returning an empty chunk; in this case, poll_next should return Pending.

[mzabaluev]

This has been made obsolete by 94b894a.