Migrate from packed_simd to stdlib simd (aka portable simd)

[SUPERCILEX]

Packed simd stopped compiling a few days ago: rust-lang/packed_simd#343. It should be replaced with portable simd: https://doc.rust-lang.org/std/simd/index.html

[bjorn3]

This issue is preventing rapier from working with cg_clif when simd-unstable is enabled: https://github.com/bjorn3/rustc_codegen_cranelift/issues/1361

[davemollen]

packed_simd stopped compiling because the platform_intrinsics feature has been removed. Maybe this makes the earlier request for replacement with portable simd relevant again.

See the issue at the packed_simd repository: rust-lang/packed_simd#360

[ProfFan]

And https://github.com/rust-lang/portable-simd