SI-8835 Lazier slice for Iterator

An iterator for slicing that does not drop eagerly.

A series of `take m drop n` that results in nothing taken
will not call the underlying iterator.

Chained invocations do not create intermediate iterators.

This PR against 2.12 (because of the change to laziness)
includes the expanded unit test from #4075, but does not
delete the redundant test files.

Fix the parallel iterable splitter, which overrides `take`
and `slice`, to also override `drop`. Drop is now eager
there, like the drop of slice.

Enhance `stringOf` to handle `ParIterable` again. A commit
from a previous decade added that under the aegis of `Iterable`.
As of 2.11, `stringOf((0 until 512).to[Vector].par, 10)`
does not truncate.