Introduce an EncodedBytes combinator which encodes multiple ready
messages - up to a yield threshold - before yielding the next bytes
buffer.

Or, if the message stream polls to pending, yield the available bytes
immediately.

These ammortized buffers exhibit far better throughput when streaming
a high rate of small messages, because hyper and h2 avoid copying the
yielded buffer and dispatch each as a separate, non-vectorized tcp send.