StreamConverters toScala: add type class for collections with efficient merging

[lrytz]

In 2.13, scala.jdk.StreamConverters adds a toScala extension method to streams that allows converting a stream to arbitrary Scala collections.

Collecting a parallel stream is especially efficient when the target collection supports efficient merging.

Currently the toScala method is special-cased for the Accumulator factory. It would make sense to generalize the ad-hoc AccumulatorFactoryInfo into a type class for collections that support efficient merging.

[Ichoran]

@lrytz - It probably would be good, but we have approximately zero collections that have efficient merging. (The only one I can think of offhand is ListBuffer, and it doesn't split efficiently, so it's kind of a dead end for stream processing.) So I don't think it's too critical right now.