Introduce ready_chunks adaptor

[glebpom]

This is the implementation of ready_chunks adapter. It is similar to chunks, but collects the data while the underlying stream returns Poll::Ready. Otherwise, the collected vector immediately returns so that the stream never blocks. The motivation behind it is that often we need to group multiple elements from the stream (specifically mpsc channels) and send them in a single system call. For example, it may be usable with calls like sendmmsg or UDP GSO

[cramertj]

This looks great to me, thanks!

[cramertj]

This looks great to me, thanks! I'd also be interested in a stream.read_next_ready_chunk(&mut buffer) or similar that would read chunks into a preallocated vector to avoid the repeated heap allocations every time.