- remove `read_stream` and `io_stream` as Lwt_stream is difficult to
  combine with error handling
- remove `read_until` and only expose the line handling functions.
  I'm also considering moving the line-handling functions into a
  separate signature since most consumers of CHANNEL only want the
  buffering logic.
- no more exceptions, just use `result` and propagate `FLOW.error`
  in return values.

[WIP] Refactor CHANNEL types to be exception-less and use `result`