MontyForm get MOD from instance

[Fethbita]

When having access to a value in ConstMontyForm, getting the MODULO value is difficult. This is the only way I could think of:

use crypto_bigint::{CheckedAdd, U256, Zero, impl_modulus, modular::ConstMontyForm};

impl_modulus!(
    MyModulus,
    U256,
    "73eda753299d7d483339d80809a1d80553bda402fffe5bfeffffffff00000001"
);
pub type ValueUnderMod = ConstMontyForm<MyModulus, { U256::LIMBS }>;

fn main() {
    // ...
    let some_value = ValueUnderMod::new(&U256::from_u8(2));

    let mut zero = some_value.clone();
    zero.set_zero();

    let mut modulo = zero.sub(&ValueUnderMod::new(&U256::from_u8(1))).retrieve();
    modulo = modulo.checked_add(&U256::from_u8(1)).unwrap();

    assert!(modulo.eq(&U256::from_be_hex(
        "73eda753299d7d483339d80809a1d80553bda402fffe5bfeffffffff00000001"
    )))
}

And it would be good to have a method for the MontyForm to get the modulus from it.

[tarcieri]

I now notice we're missing an impl of the Monty trait for ConstMontyForm. Never mind, that won't work in the current form

I think an associated inherent constant might make sense here, like pub const MODULUS: Odd<Uint<LIMBS>>?

If you really need to get it through the value rather than e.g. ValueUnderMod::MODULUS, we can also add an impl of the Monty trait. perhaps we can consider a method too, though it would be nice if it were part of a trait

[tarcieri]

@Fethbita does #982 address your issue?

[Fethbita]

Yes, this fixes the issue. It's okay to have the constant for this. Method looks like it could make more sense but still, not a problem. Thank you very much.

[tarcieri]

I think a method could make sense in the context of a trait that can also be impl'd by MontyForm and BoxedMontyForm