Macro dispatch_light128

macro_rules! dispatch_light128 {
    ($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 128-bit vectors on this platfrom. For x86-64, that would mean SSE2 and AVX.

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.