Issues in new I/O

[mzabaluev]

@mzabaluev

The contract of flush does not fit all use cases. It's specified as a top-to-bottom flush, but methods like into_inner on composed writers call it before returning the underlying writer that you may want to continue writing to. It may be preferred in some cases (e.g. stateful encoding writers) to only finalize output on the topmost layer.

In fact, this is the only motivation I have for a "shallow flush" at the moment, and this can be addressed by changing the contract on into_inner to only finalize writing, but not flush the returned writer. With an implication that the consumer should take care of the eventual flushing.