Add methods for storing vectors back to memory

[LaurenzV]

As per title, by having this, we don't have to store them back using copy_from_slice (which may or may not optimize into vector store instructions) but can use methods that use the corresponding intrinsics under the hood.

[LaurenzV]

(Will add changeling entry upon approval.)

[valadaptive]

This looks good! Just a few minor code style nits.

[valadaptive]

Maybe this should be renamed StoreArray?

[LaurenzV]

Thanks! By the way, #171 has also been approved in case you haven't seen it. I could probably take another look at #164 if you want.

[Shnatsel]

Note that storing to fixed-size slices involves a massive footgun: https://github.com/okaneco/safe_unaligned_simd?tab=readme-ov-file#a-note-on-creating-mutable-array-references-from-slices

This isn't a problem for loads, only for stores. I've been pulling my hair for an hour debugging something that absolutely should work but didn't because of this footgun, and that's how it came to be documented in that repo's README.

The best way to avoid it that I can think of is to make the output slice a &mut [T] as opposed to &mut [T; N] and include an assert! checking the length with >= in each function. All of this is #[inline(always)] so if the length is known to be sufficient the check will be eliminated.

[valadaptive]

Note that storing to fixed-size slices involves a massive footgun

If you use the store_slice method, you should not be subject to this footgun.

The only way to fall victim to this footgun in fearless_simd is if you use the store_array_[whatever] methods directly, like vector.witness().store_array_f32x4(vector, &mut chunk[..4].try_into().unwrap()).

[Shnatsel]

I didn't realize that was an option. If there's already a store_slice, do we even need the footgunny store_array_blah() functions?

I think they should either be removed, or at the very least footgun should be mentioned in their doc comment.

[LaurenzV]

I think at least internally we still need the methods... And making it private would be a bit of a pain I think.

[Shnatsel]

Well, if they can't be made pub(crate) then I think it's worth at least documenting the footgun if we can't prevent it