* Fixed `PublisherCoroutine` and `rxObservable` ignoring
  cancellations.
* Fatal exceptions are not treated in a special manner by us
  anymore. Instead, we follow the requirement in the reactive
  streams specification that, in case some method of `Subscriber`
  throws, that subscriber MUST be considered cancelled, and the
  exception MUST be reported in some place other than `onError`.
* Fixed `trySend` sometimes throwing in `PublisherCoroutine` and
  `rxObservable`.
* When an exception happens inside a cancellation handler, we now
  consistently throw the original exception passed to the handler,
  with the new exception added as suppressed.

* Fixed `doLockedNext` not releasing the lock in
  `PublisherCoroutine` if `null` is emitted
* Fixed `flux`, `publish`, `rxObservable` and `rxFlowable`
  incorrectly reporting `isClosedForSend == true` just after
  closing the channel