Macro dispatch_light256

macro_rules! dispatch_light256 {
    ($mach:ident, $MTy:ident, { $([$pub:tt$(($krate:tt))*])* fn $name:ident($($arg:ident: $argty:ty),* $(,)*) -> $ret:ty $body:block }) => { ... };
    ($mach:ident, $MTy:ident, { $([$pub:tt$(($krate:tt))*])* fn $name:ident($($arg:ident: $argty:ty),* $(,)*) $body:block }) => { ... };
}

Generate only the basic implementations necessary to be able to operate efficiently on 256-bit vectors on this platfrom. For x86-64, that would mean SSE2, AVX, and AVX2.

This dispatcher is suitable for vector operations that do not benefit from advanced hardware features (e.g. because they are done infrequently), so minimizing their contribution to code size is more important.