Undeprecate Channel.receiveOrNull

[elizarov]

• It is a valid method for channels of non-null elements that is
  also consistent with Kotlin stdlib tradition.
• However, leave Channel.onReceiveOrNull since this is a tricker one,
  because on failure it aborts the whole select without letting figure
  out which clause failed.

[jcornaz]

What about having this kind of function instead?

suspend inline fun <E> ReceiveChannel<E>.receiveOr(action: () -> E): E {
  return try {
    receive()
  } catch (closedException: ClosedReceiveChannelException) {
    action()
  }
}

This let the user choose what to do if the the channel is closed.

val nullableValue = channel.receiveOr { null }
val nonNullableValue1 = channel.receiveOr { return }
val nonNullableValue2 = channel.receiveOr { throw Exception() }

[elizarov]

@qwwdfsad Please, review and merge for 1.0.0 release

[qwwdfsad]

Fixed in develop.
Deprecation is removed, but "obsolete" marker is not.