Fix: Allow struct fields to propagate constants

[katat]

This is a fix to propagate the constant values from struct fields also.

This fix allows the struct fields to be passed as an constant argument for a generic function. eg:

struct Uint8 {
    inner: Field,
    bit_len: Field,
}

fn Uint8.new(val: Field) -> Uint8 {
    let bit_len = 8;

    // range check
    let ignore_ = bits::to_bits(bit_len, val);

    return Uint8 {
        inner: val,
        bit_len: bit_len
    };
}

fn Uint8.less_than(self, rhs: Uint8) -> Bool {
    // this won't work without this PR fix
    return comparator::less_than(self.bit_len, self.inner, rhs.inner);
}

[gio54321]

Looks good to me, I did not find any issues in the implementation, this is definitely a fix worth merging in the stdlib PR 😄

I'm wondering, defining Uint8 using a structure allows an user to modify the bit length, which maybe is not necessarily what we want. Perhaps it is better to define it using a constant BIT_LEN so that we ensure that it is fixed in the monomorphization phase.
Afaiu, if the user using Uint8 do modify the bit length with another constant, the monomorphization would still pass with the new bit length, but maybe I'm wrong.

[katat]

Indeed it is potentially an issue when it comes to the fact that the users can modify the field, such as bit_len, which supposes to be a constant and can't be changed once the struct is declared. There is an ongoing discussion on this issue: #194