Introduce semantic conversion traits

[Ralith]

This proposes a generic API for conversions between vectors of the same element size and count, usable on generic native-width vectors. If I get positive feedback I'll fill out some more cases before merging.

[Ralith]

Open question: Should this supplant Simd::cvt_*? If not, it could probably be blanket-implemented based on those methods rather than separately implemented for each backend as in this PR. I do feel like the naming of those methods is a bit confusing.

[ajakubowicz-canva]

Discussion about this proposal: https://xi.zulipchat.com/#narrow/channel/514230-simd/topic/Ergonomic.20native-width.20conversions

[Ralith]

Second draft:

• Multiple, semantically meaningful traits instead of a single overloaded "cast"
• Helper methods on SimdFloat and SimdInt

I don't like the trait or primitive method names much at all; please bikeshed. Especially "float" as a verb feels bad.

[Ralith]

Tweaked naming, factored out common generation logic, and expanded to a superset of existing conversion operations. I think this is ready now, though still happy to brainstorm on better names.

[Ralith]

Reworked to allow conversions for e.g. f32x4<S> rather than just S::f32s, by building on top of, rather then replacing, the Simd methods.

[ajakubowicz-canva]

Sorry for the lack of communication – I plan to discuss this PR and proposal at the next Renderer office hours. Thank you for your excellent work!

[ajakubowicz-canva]

Raised this PR at the Linebender Renderer Office Hours and we have consensus to move forward with names as currently written. I love the direction of this PR. We discussed not worrying too much about bike-shedding as fearless_simd is still very early and we can always make a breaking change in these early days if we find something doesn't work well for us.

One minor complexity is that I think you need to rebase this PR and make sure that the code generator output is updated to reflect the latest main branch.

Thank you thank you!

[Ralith]

Rebased and regenerated.

[Ralith]

Whoops, needs some tweaks to handle i64.

[Ralith]

We don't actually have u/i64xN yet, so I've just masked those cases off for now.

[LaurenzV]

Thanks!