Unable to call .iterator on an IterableOnceOps value

[julienrf]

See this contributors discussion for the motivation.

In summary, using IterableOnceOps would make it possible to write code polymorphic to Iterable or Iterator, but methods defined by IterableOnce (such as iterator) are not available on IterableOnceOps values.

We should add abstract definitions of the methods declared in IterableOnce, to the IterableOnceOps type.

[NthPortal]

this is a good candidate for scala/scala-library-next