Scala 2.13 support

[michaelmior]

Thanks for this library! Wondering if it's possible to get Scala 2.13 support. It looks like this should be fairly straightforward since the completion of #2.

[michaelmior]

My mistake! I was unaware that parallel collections were removed in Scala 2.13. It seems as though this library adds back support for parallel collections. However, it does not appears to support parallel iterators. Mistaken again. However, I'm guessing that iterata can't be easily made to support both the Scala standard library in earlier versions and the library I referenced above.

[michaelmior]

Apologies for all the repeated comments. But I did notice there are folks who seem to have cross builds working. I tried some of this myself and some tests fail in Scala 2.13, but it seems like this is at least possible, so I'll reopen for the time being.