Use scala-parallel-collections library in 2.13

Classes like ForkJoinTaskSupport and .par moved to scala-parallel-collections in 2.13. This needs to be included as a dependency only in 2.13 via a profile. However we'll also have to rewrite use of .par to get this to work in 2.12 and 2.13 simultaneously:

https://github.com/scala/scala-parallel-collections/issues/22