(Taken on a x86_64 macbook 2.9 GHz Intel Core i9 with 6 cores)
Where `is_ascii_slice_iter_all` is the old version, and `is_ascii_slice_libcore` is the new.
I tried to document the code well, so hopefully it's understandable. It has fairly exhaustive tests ensuring size/align doesn't get violated -- because `miri` doesn't really help a lot for this sort of code right now, I tried to `debug_assert` all the safety invariants I'm depending on. (Of course, none of them are required for correctness or soundness -- just allows us to test that this sort of pointer manipulation is sound and such).
Anyway, thanks. Let me know if you have questions/desired changes.