Don't export `reindex` and `substrides` (#31201)
* Don't export `reindex`
It was inadvertently exported by #30789. It's tailored to `SubArray` and doesn't check for preconditions that are enforced by the `SubArray` constructor — so while we could export it, I'd like to do so intentionally and add friendlier errors for violations of these preconditions.
* Same deal for `substrides`
