SIMD-style optimizations are the most common use for `[T]::align_to(_mut)`, but that's `unsafe`.  So these are *safe* wrappers around it, now that we have the `Simd` type available, to make it easier to use.

```rust
impl [T] {
    pub fn as_simd<const LANES: usize>(&self) -> (&[T], &[Simd<T, LANES>], &[T]);
    pub fn as_simd_mut<const LANES: usize>(&mut self) -> (&mut [T], &mut [Simd<T, LANES>], &mut [T]);
}
```