Function is_ascii 

const fn is_ascii(bytes: &[u8]) -> bool

Available on AArch64 and target feature neon, or x86-64 and target feature sse2 only.

Uses explicit SIMD intrinsics to prevent LLVM from auto-vectorizing with broken code (e.g., AVX-512 on x86-64 that extracts mask bits one-by-one).