Move Field operations to operator-backed traits

The ff_derive, pairing, zcash_primitives::jubjub, and bellman dummy_engine
changes are minimally implemented on top of the existing *_assign()
functions.

zcash_proofs/src/circuit/ecc.rs

@@ -2,6 +2,7 @@
 
 use ff::Field;
 use pairing::Engine;
+use std::ops::{AddAssign, MulAssign, SubAssign};
 
 use bellman::{ConstraintSystem, SynthesisError};
 
@@ -668,6 +669,7 @@ mod test {
     use pairing::bls12_381::{Bls12, Fr};
     use rand_core::{RngCore, SeedableRng};
     use rand_xorshift::XorShiftRng;
+    use std::ops::SubAssign;
 
     use bellman::gadgets::test::*;
     use zcash_primitives::jubjub::fs::Fs;

zcash_proofs/src/sapling/prover.rs

@@ -5,6 +5,7 @@ use bellman::{
 use ff::Field;
 use pairing::bls12_381::{Bls12, Fr};
 use rand_core::OsRng;
+use std::ops::AddAssign;
 use zcash_primitives::{
     jubjub::{edwards, fs::Fs, FixedGenerators, JubjubBls12, Unknown},
     primitives::{Diversifier, Note, PaymentAddress, ProofGenerationKey, ValueCommitment},