Support a parallel iterator getting more work to do from a normal iterator

[golddranks]

The motivation is as follows: I'm getting asynchronously produced values from a channel. For each value, there is some independent, CPU-heavy work to do. I want to maximise the resource use, and do the work in parallel if there is CPU power available and there's values available from the channel.

The types I: Iterator<Item=T> and/or Receiver<T> don't implement the IntoParallelIterator trait. Is there any reason for that?

[garro95]

Maybe similar to #46

[cuviper]

Yes, let's track it in #46.