[DSLX] In struct parametrics parametric argument types other than `u32` should be allowed, including `s32`

**Describe the bug**

This does not work:
```
pub struct FixedPoint {
data: uN[NUM_BITS],
}
#[test]
fn test_neg_exp() {
let a = FixedPoint { data: u8:0 };
}
```

because BINARY_EXPONENT is s32. It works if I change it to u32. I really do want a signed value, so this is not acceptable.

Error message, referring to BINARY_EXPONENT when I create `a`:
```
sN[32] vs uN[32]: Dimension s32:-1 must be a `u32` (soon to be `usize`, see https://github.com/google/xls/issues/450 for details).
```

Note that other parametric argument types which are useful to implementers are also disallowed, e.g. `bool` would have been useful to me to make a signed magnitude workaround:

```
pub struct FixedPoint
```

but the above is also not allowed.

**To Reproduce**

Use the code above, try to compile.

**Expected behavior**

That I can make BINARY_EXPONENT signed without issues.

This would be very handy for making an open source fixed point library.