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).

bellman/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};