Macro hektor::if_sse42

macro_rules! if_sse42 {
    ($yes:block else $no:block) => { ... };
}

Use this to make it simpler to handle the branch between using sse4.2 or not.

The SIMD path is automatically wrapped in unsafe. Also, the cargo feature debug_force_no_sse42 will cause this macro to pick the non-sse4.2 path even if the CPU supports it. Use this for easy testing of the fallback path.