Macro impl_to_bytes

macro_rules! impl_to_bytes {
    { $ty:tt, 1  } => { ... };
    { $ty:tt, 2  } => { ... };
    { $ty:tt, 4  } => { ... };
    { $ty:tt, 8  } => { ... };
    { $ty:tt, 16 } => { ... };
    { $ty:tt, 32 } => { ... };
    { $ty:tt, 64 } => { ... };
    { $ty:tt, $size:literal * [$($elems:literal),*] } => { ... };
}

🔬This is a nightly-only experimental API. (portable_simd #86656)