Saturating casts for integers, like `_mm_packus_epi16` and `_mm_packs_epi16`

My most common use case is for saturating narrowing casts, such as from i16 to u8. It would be great to take advantage of native instructions without first having to use simd_clamp. saturating_cast would reduce code and chances of errors from clamping with incorrect values.

I'm not sure about the availability of this feature on other architectures. It might make sense to split this for narrowing and widening features based on availability. I'm not familiar with other architectures, but it looks like ARM has narrowing casts with VQMOVN/VQMOVUN.

Perhaps there could be a fallback which does simd_clamp().cast() (or other optimal construction) for architectures that don't have these instructions.

Start by reviewing the existing simd_clamp and cast APIs, then investigate the availability of _mm_packus_epi16, _mm_packs_epi16, and ARM VQMOVN/VQMOVUN across supported architectures. The work is done when saturating narrowing casts have a defined architecture strategy, including an appropriate fallback where native instructions are unavailable.