Const array SoA variant

I think it would be nice to generate a type like:

```rust
pub struct FooArray<const N: usize> {
    pub f1: [f32; N],
    pub f2: [u8; N],
    ...
}
```

This would allow you to do stuff like `core::alloc::Layout::new::<FooArray<128>>()` for free.