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/jubjub/tests.rs

@@ -1,6 +1,7 @@
 use super::{edwards, montgomery, JubjubEngine, JubjubParams, PrimeOrder};
 
 use ff::{Field, LegendreSymbol, PrimeField, PrimeFieldRepr, SqrtField};
+use std::ops::{AddAssign, MulAssign, SubAssign};
 
 use rand_core::{RngCore, SeedableRng};
 use rand_xorshift::XorShiftRng;