Add as_array to SimdValue?

Often it would be convenient to be able to cast the entire SIMD value to an array reference instead of extracting lanes one by one, in a generic code over any `SimdValue`.

As far as I can tell, all implementations of `SimdValue` should be able to support this - `AutoSimd<[T; N]>` has array easily accessed via `.0`, primitives can be casted via `std::slice::from_ref`, `num_complex::Complex` has `repr(C)` compatible with `[T; 2]`, `std::simd`-based implementation has its own `as_array`, and `wide`-based implementation has `as_array_ref`.

With that in mind, can we add `as_array` to `SimdValue` trait itself to make writing generic code easier?