Add the small subgroup power for BN254 (#163)

Co-authored-by: mmagician <marcin.gorny.94@protonmail.com>
Co-authored-by: Pratyush Mishra <pratyushmishra@berkeley.edu>

bn254/src/fields/fr.rs

@@ -3,5 +3,7 @@ use ark_ff::fields::{Fp256, MontBackend, MontConfig};
 #[derive(MontConfig)]
 #[modulus = "21888242871839275222246405745257275088548364400416034343698204186575808495617"]
 #[generator = "5"]
+#[small_subgroup_base = "3"]
+#[small_subgroup_power = "2"]
 pub struct FrConfig;
 pub type Fr = Fp256<MontBackend<FrConfig, 4>>;