simd.h cleanup (#1475)
simd.h cleanup (#1475)

* Macros to un-4-ify the generic code. This reduces some clutter and
  will makes it much easier to clone these classes for 8-wide and 16-wide
  in the future.

* Define simd_raw_t<N> and simd_bool_t<N> tempaltes that make it easier
  to reduce a bit of the #if clutter.
OpenImageIO/oiio,YangYangTL/oiio,lgritz/oiio,YangYangTL/oiio,OpenImageIO/oiio,OpenImageIO/oiio,OpenImageIO/oiio,lgritz/oiio,YangYangTL/oiio,YangYangTL/oiio,lgritz/oiio,lgritz/oiio