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

[dank-openai]

Describe the bug

This does not work:

pub struct FixedPoint<NUM_BITS: u32, BINARY_EXPONENT: s32> { 
    data: uN[NUM_BITS], 
}
#[test]
fn test_neg_exp() {
    let a = FixedPoint<u32:2, s32:-1> { 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<NUM_BITS: u32, EXPONENT_IS_NEGATIVE:bool, BINARY_UEXPONENT: u32>

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.

[erinzmoore]

Taking a look at this, but given the state of type system v2, if this proves to be very involved, the fix may wait for v2. Oh, re-reading this I see it isn't specific to impl, but to parametric types in all cases. So, this will in fact be blocked on type system v2. Unassigning