std.simd module

[Martin Nowak]

> There must be a library that provides the opportunity for the best
> performance, before sugary libs get all layered over the top, and it's as
> I've said twice now, everyone will have a different idea of what that  
> sugar
> API will look like, so I feel it should live above this... or perhaps
> beside this (in the same file?), but I wouldn't want to remove this API  
> in
> favour of a vector 'class'.

Having those generalized intrinsics publicly available in same file  
probably makes sense.