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_primitives/src/pedersen_hash.rs

@@ -2,6 +2,7 @@
 
 use crate::jubjub::*;
 use ff::{Field, PrimeField, PrimeFieldRepr};
+use std::ops::AddAssign;
 
 #[derive(Copy, Clone)]
 pub enum Personalization {