ff: Move pow_vartime back into Field trait

The only places we don't use constant u64 limbs, we use PrimeField::char
instead (except in a single test where we use a field element).

src/domain.rs

@@ -11,7 +11,7 @@
 //! [`EvaluationDomain`]: crate::domain::EvaluationDomain
 //! [Groth16]: https://eprint.iacr.org/2016/260
 
-use ff::{Field, PowVartime, PrimeField, ScalarEngine};
+use ff::{Field, PrimeField, ScalarEngine};
 use group::CurveProjective;
 use std::ops::{AddAssign, MulAssign, SubAssign};
 

src/gadgets/multieq.rs

@@ -1,4 +1,4 @@
-use ff::{PowVartime, PrimeField, ScalarEngine};
+use ff::{Field, PrimeField, ScalarEngine};
 
 use crate::{ConstraintSystem, LinearCombination, SynthesisError, Variable};
 

src/gadgets/test/mod.rs

@@ -1,6 +1,6 @@
 //! Helpers for testing circuit implementations.
 
-use ff::{Endianness, Field, PowVartime, PrimeField, ScalarEngine};
+use ff::{Endianness, Field, PrimeField, ScalarEngine};
 
 use crate::{ConstraintSystem, Index, LinearCombination, SynthesisError, Variable};
 

src/groth16/generator.rs

@@ -2,7 +2,7 @@ use rand_core::RngCore;
 use std::ops::{AddAssign, MulAssign};
 use std::sync::Arc;
 
-use ff::{Field, PowVartime};
+use ff::Field;
 use group::{CurveAffine, CurveProjective, Wnaf};
 use pairing::Engine;
 

src/groth16/tests/dummy_engine.rs

@@ -1,4 +1,4 @@
-use ff::{Field, PowVartime, PrimeField, ScalarEngine};
+use ff::{Field, PrimeField, ScalarEngine};
 use group::{CurveAffine, CurveProjective, EncodedPoint, GroupDecodingError};
 use pairing::{Engine, PairingCurveAffine};
 

src/groth16/tests/mod.rs

@@ -1,4 +1,4 @@
-use ff::{Field, PowVartime, PrimeField};
+use ff::{Field, PrimeField};
 use pairing::Engine;
 
 mod dummy_engine;