SI-8924 No outer ref in LinearSeq iterator

The iterator for `LinearSeqLike` holds an outer pointer via
the call to `newBuilder`, which is used to empty the underlying
sequence on `toList`.

A better strategy is to use `these take 0` to create the empty
`Seq`.  We can rely on `take 0` to mean the empty `Repr`.

The unit test confirms that a weak reference to the sequence
is collectable after `seq.iterator.toList`.
That's an interesting suggestion.  It's harmless, but it will also have to guard against interruption; though if the allocator gets in one more allocation and it fails, even though the test has already passed, is that itself a failure?  Rhetorical question.  I'm not sure there is a notion of "next test", since usually you expect tests in parallel. But it's about ensuring clean-up.