simd_cast: documentation + no arg overload + enable_if move
simd_cast: documentation + no arg overload + enable_if move

* add new simd_cast that implements cast from nothing (useful for
  generic code that uses variadic templates (empty pack expansion))
* document the two simple simd_cast functions
* move enable_if to return value instead of hidden argument

VcDevel/Vc,chr-engwer/Vc,chr-engwer/Vc,chr-engwer/Vc,VcDevel/Vc,VcDevel/Vc,VcDevel/Vc