[dslx] Allow struct parametrics to be types other than `u32`

Right now we evaluate all parametric expressions given to a struct as `u32`s, but that's technically too strict a limitation -- although we only allow u32s to appear in e.g. array dimensions when building a type, you could imagine a use case like the following:

```dslx
struct Point {
x: uN[N_U32],
}

fn f(p: Point) -> uN[3] {
p.y
}
```

Note that the value given as N is not used in any type definition, just as a value that we do some computation from.

Note that I'm going to land a change that overly restricts them to all be `u32`s at first to fix #727, then we can loosen from there.