Make Empty work faster with the SlippyIterator
Make Empty work faster with the SlippyIterator

- check for Empty directly at startup
- check for first value pulled, don't start if IterationEnd:
  this should handle the slip() cases faster
- rewrite method in nqp entirely
- 5% faster in my @a = ^1000; for ^1000 { my @b = @a.grep({ 0 }) }

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