halo2_gadgets: Fix return type of `ecc::chip::FixedPoint::u`

This trait method was previously preventing the trait to working with
fields that encode to more than 256 bits.

halo2_gadgets/CHANGELOG.md

@@ -49,6 +49,8 @@ and this project adheres to Rust's notion of
   - `FixedPointShort::mul` now takes `ScalarFixedShort` instead of
     `(EccChip::Var, EccChip::Var)`.
 - `halo2_gadgets::ecc::chip`:
+  - `FixedPoint::u` now returns `Vec<[<C::Base as PrimeField>::Repr; H]>`
+    instead of `Vec<[[u8; 32]; H]>`.
   - `ScalarKind` has been renamed to `FixedScalarKind`.
 - `halo2_gadgets::sinsemilla`:
   - `CommitDomain::{commit, short_commit}` now take the trapdoor `r` as an

halo2_gadgets/src/ecc/chip.rs

@@ -7,7 +7,7 @@ use crate::{
 };
 use arrayvec::ArrayVec;
 
-use ff::Field;
+use ff::{Field, PrimeField};
 use group::prime::PrimeCurveAffine;
 use halo2_proofs::{
     circuit::{AssignedCell, Chip, Layouter},
@@ -210,7 +210,7 @@ pub trait FixedPoint<C: CurveAffine>: std::fmt::Debug + Eq + Clone {
     fn generator(&self) -> C;
 
     /// Returns the $u$ values for this fixed point.
-    fn u(&self) -> Vec<[[u8; 32]; H]>;
+    fn u(&self) -> Vec<[<C::Base as PrimeField>::Repr; H]>;
 
     /// Returns the $z$ value for this fixed point.
     fn z(&self) -> Vec<u64>;