SI-9913 Lead span iterator finishes at state -1

[som-snytt]

Even if no elements fail the predicate (so that the trailing
iterator is empty).

Review by @Ichoran

JIRA: https://issues.scala-lang.org/browse/SI-9913

[som-snytt]

I noticed leading.status goes toward negative, while trailing.status goes toward positive.

It would be handy if one could introduce a private trait of constant value defs (inlined constants) and then for the compiler not to emit the trait class. I would name the constants if it were free. Or if it didn't emit the field at least for private[this] final val K = 42.

[szeiger]

LGTM

[SethTisue]

thanks Som!

[Ichoran]

@som @adriaanm - Sorry, still swamped. This one slipped through the cracks. I am not sure how you get cracks in a swamp, but there you go.