Remove seqStream / parStream from Stepper subclasses
Remove seqStream / parStream from Stepper subclasses

For two reasons:
  - parStream should only be available if there's EfficientSubstep
  - the Stepper API should not refer to Stream, only to Spliterator and
    java.util.Iterator, to minimize the dependency

The seqStream / parStream methods will come back as extension methods.

scala/scala,lrytz/scala,scala/scala,lrytz/scala,lrytz/scala,martijnhoekstra/scala,lrytz/scala,scala/scala,scala/scala,martijnhoekstra/scala,scala/scala,martijnhoekstra/scala,martijnhoekstra/scala,lrytz/scala,scala/scala,martijnhoekstra/scala,martijnhoekstra/scala,lrytz/scala