Provide CoroutineScope receiver to flowViaChannel lambda

[qwwdfsad]

Original request: #254 (comment)

Additionally, we should discuss whether it will be CoroutineScope or ProducerScope and its relation with not yet discusses scopedFlow

[qwwdfsad]

After some thoughts, we've decided to provide CoroutineScope receiver, but remove suspend modifier from lambda, thus replacing the signature block: suspend (SendChannel<T>) -> Unit with block: CoroutineScope.(SendChannel<T>) -> Unit.

Why so? I see two possible usages of this API:

• Integrating with callback-based API as in our KDoc sample. For this reason, one doesn't need suspension (it is even harmful to Java interoperability). CoroutineScope is not required either, but it is present in the implementation anyway (and we cannot get rid of it) and it provides additional safety mechanism as there is no temptation to use GlobalScope

• Integration with blocking API, but from the coroutine world. For example:

flowViaChannel {
  withContext(IO) {
     channel.send(doBlockingCall())
  }
}

While this looks reasonable, it is not the most idiomatic way. Instead, flow builder is much more suitable:

flow {
  emit(doBlockingCall())
}.flowOn(IO)

cc @jcornaz as the author of the original request