Zeroize performance on u8 arrays

I inspected the generated assembly code and benchmarked zeroize for [u8; 32] on x86_64 and found it quite inefficient, storing one byte at a time:

https://play.rust-lang.org/?version=stable&mode=release&edition=2021&gist=3f44f4b90e6af0eac0dcb1f649390329

On my Ryzen CPU, it takes ~7.8324 ns, or ~1cpb. Binary code size is also quite large.

Using inline assembly (just stabilized in 1.59) and SSE2, zeroing a [u8; 32] takes just 3 instructions and ~492.87 ps (~16 bytes per cycle):

let mut buf: [u8; 32];
core::arch::asm!(
    "xorps {zero}, {zero}",
    "movups {zero}, ({ptr})",
    "movups {zero}, 16({ptr})",
    zero = out(xmm_reg) _,
    ptr = in(reg) &mut buf,
    options(att_syntax, nostack, preserves_flags),
);

So it might be something worth optimizing/documenting.

If you do not want to use inline assembly, maybe you should encourage using larger types or SIMD types, e.g., [u64; 4] or [__m128; 2] instead of [u8; 32]. Using write_volatile on *mut __m128 generates equally compact and efficient code as the assembly code above.

Start with the Rust Playground benchmark linked in the issue and inspect the generated assembly for zeroize on [u8; 32]. Compare the current output with the inline-assembly and larger-type approaches described in the report. Done requires an agreed optimization or documentation change, with its performance and code-size impact verified.