Structured bindings for all data parallel types

[bernhardmgruber]

Although the proposed data parallel types are conceptually not arrays, they still provide the subscript operator and can at least be treated similarly to arrays. A notable missing piece is the ability to destructure data parallel types, possibly into packs:

int main() {
    auto s = std::experimental::native_simd<double>{[](auto ic) -> float { return ic * 2; }};
    auto& [a, b, c, d] = s; // only with e.g. AVX2
    const auto [...parts] = s; // generic, P1061
}

The implementation should not be too hard, here is a sketch: https://godbolt.org/z/zGYx1vezv