implement SubArray reindexing without using `@generated` (#58800)
The tuple specialization for indexing with UnitRanges is highly
optimized, type stable, and constant foldable for statically known
values like these.
Seems like a very good thing to just lean on that. It's how I would've
written this in the first place had that optimized method existed when I
wrote it. It's much more readable and — I imagine — it should be quite a
bit friendlier to the compiler. The only time you should see a
performance difference here is when you have a SubArray with 11 or more
indices **and** one of those indices is 3-dimensional or more.
