Rollup merge of #74066 - thomcc:optimize-is-ascii, r=nagisa

Optimize is_ascii for str and [u8].

This optimizes the `is_ascii` function for `[u8]` and `str`. I've been surprised this wasn't done for a while, so I just did it.

Benchmarks comparing before/after look like:

```
test ascii::long_readonly::is_ascii_slice_iter_all              ... bench:         174 ns/iter (+/- 79) = 40172 MB/s
test ascii::long_readonly::is_ascii_slice_libcore               ... bench:          16 ns/iter (+/- 5) = 436875 MB/s
test ascii::medium_readonly::is_ascii_slice_iter_all            ... bench:          12 ns/iter (+/- 3) = 2666 MB/s
test ascii::medium_readonly::is_ascii_slice_libcore             ... bench:           2 ns/iter (+/- 0) = 16000 MB/s
test ascii::short_readonly::is_ascii_slice_iter_all             ... bench:           3 ns/iter (+/- 0) = 2333 MB/s
test ascii::short_readonly::is_ascii_slice_libcore              ... bench:           4 ns/iter (+/- 0) = 1750 MB/s
```

(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.

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