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.

pairing/benches/bls12_381/fr.rs

@@ -1,5 +1,6 @@
 use rand_core::SeedableRng;
 use rand_xorshift::XorShiftRng;
+use std::ops::{AddAssign, MulAssign, SubAssign};
 
 use ff::{Field, PrimeField, PrimeFieldRepr, SqrtField};
 use pairing::bls12_381::*;