Expose Fr::MODULUS

[znewman01]

As jubjub::MODULUS:

use jubjub::{Fr, MODULUS};

let modulus: Fr = MODULUS;
// do something with it

---

IMO the current docs ("Constant representing the modulus r = 0x...") are okay, and it doesn't need a rename or anything.

I found this useful, not sure if others will.

[str4d]

The comment https://github.com/zkcrypto/bls12_381/pull/39#issuecomment-617994330 applies here:

The reason this is not exposed is because it's not an element of the field, so it breaks the contract of the Scalar type. We should expose the underlying [u64; 4] representation though.

If you change your PR to expose the [u64; 4] portion of the modulus instead, and replace MODULUS with Scalar(MODULUS) elsewhere in the file, that would be fine.

[znewman01]

Makes sense; just force-pushed.

Couple notes:

• the constant in fr.rs was 4 64-bit words; lib.rs has a 32-byte constant that I moved into fr.rs and ultimately exposed.
• I kept its lib.rs name FR_MODULUS_BYTES---not totally sure how to reconcile Fr vs. Scalar

[znewman01]

@str4d

anything else to do here?

[znewman01]

@str4d ping

[ebfull]

Hey! This change is being done (indirectly) as part of a cross-crate refactoring effort happening in this repository. So, soon this will make its way into the API somehow. The changes are slightly different, though, and I want to avoid messy conflicts.

[znewman01]

Sweet, looking forward! Feel free to close out in favor of that