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.

bellman/src/groth16/prover.rs

@@ -1,5 +1,5 @@
 use rand_core::RngCore;
-
+use std::ops::{AddAssign, MulAssign};
 use std::sync::Arc;
 
 use futures::Future;