Add Simd::cast

[workingjubilee]

We've gotten a few requests for some kind of cast, and I needed a reason to force myself to work through learning about 2FA hardware keys. I know there's been some discussion about what exactly to do, but I believe we were drifting in the direction of providing this function?

Also, thanks @calebzulawski for implementing the saturating intrinsic!

[calebzulawski]

Could you add a test for this? I know it uses the same conversion code as as, but I'd like to guarantee it.

[programmerjake]

you could remove round_from_int afaict because it's functionality is completely supplanted by cast

[programmerjake]

also to_int_unchecked should be generic over return type

[workingjubilee]

Yes, I was thinking "hm, do I want to touch fn round_from_int?" but then I realized I didn't have a good vibe anymore for how strong the consensus was around fn round_from_int and fn to_int_unchecked, so I was going to bring that up at our next sync.

[programmerjake]

ok, let's just merge this PR as is then, and handle to_int_unchecked and round_from_int in separate PR(s).