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`.
This class looks dangerous, since it can't be run in parallel with other tests and also will generate heap dump if anyone run test suite with `-XX:+HeapDumpOnOutOfMemoryError` flag. I think you could allocate a large number of larger test samples (like I did in my test) and then do `System.gc() / refq.remove()` loop that awaits for garbage collection of any first sample.