Add FieldBits impl for Secp256r1 & remove utils module (#123)

* chore: Move fe_from_str to integr tests

It doesn't make much sense to have a `utils` module which in reality
just has one function that only integration tests require.

So, chore: Move `fe_from_str` to integration tests file removing code
from the lib itself.

* fix: Include field_bits! macro impl for Secp256r1/Fq

src/lib.rs

@@ -13,8 +13,6 @@ pub mod secp256k1;
 pub mod secp256r1;
 pub mod secq256k1;
 
-pub mod utils;
-
 #[macro_use]
 mod derive;
 

src/secp256r1/fq.rs

@@ -118,7 +118,7 @@ const ZETA: Fq = Fq::from_raw([
 const DELTA: Fq = Fq::from_raw([0x1e39a5057d81, 0, 0, 0]);
 
 use crate::{
-    field_arithmetic, field_common, field_specific, impl_add_binop_specify_output,
+    field_arithmetic, field_bits, field_common, field_specific, impl_add_binop_specify_output,
     impl_binops_additive, impl_binops_additive_specify_output, impl_binops_multiplicative,
     impl_binops_multiplicative_mixed, impl_from_u64, impl_sub_binop_specify_output, impl_sum_prod,
 };
@@ -141,6 +141,11 @@ impl_from_u64!(Fq, R2);
 field_arithmetic!(Fq, MODULUS, INV, dense);
 impl_sum_prod!(Fq);
 
+#[cfg(target_pointer_width = "64")]
+field_bits!(Fq, MODULUS);
+#[cfg(not(target_pointer_width = "64"))]
+field_bits!(Fq, MODULUS, MODULUS_LIMBS_32);
+
 impl Fq {
     pub const fn size() -> usize {
         32

src/tests/curve.rs

@@ -3,10 +3,13 @@
 use crate::ff::Field;
 use crate::ff_ext::Legendre;
 use crate::group::prime::PrimeCurveAffine;
-use crate::utils::fe_from_str;
 use crate::{group::GroupEncoding, serde::SerdeObject};
 use crate::{hash_to_curve, CurveAffine, CurveExt};
+use ff::PrimeField;
+use num_bigint::BigUint;
+use num_traits::Num;
 use rand_core::{OsRng, RngCore};
+use std::borrow::Cow;
 use std::iter;
 
 #[cfg(feature = "derive_serde")]
@@ -352,6 +355,16 @@ pub fn hash_to_curve_test<G: CurveExt>() {
     }
 }
 
+fn fe_from_str<F: PrimeField>(string: impl AsRef<str>) -> F {
+    let string = string.as_ref();
+    let oct = if let Some(hex) = string.strip_prefix("0x") {
+        Cow::Owned(BigUint::from_str_radix(hex, 16).unwrap().to_string())
+    } else {
+        Cow::Borrowed(string)
+    };
+    F::from_str_vartime(&oct).unwrap()
+}
+
 pub fn svdw_map_to_curve_test<G: CurveExt>(
     z: G::Base,
     precomputed_constants: [&'static str; 4],