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/lib.rs

@@ -148,7 +148,7 @@ use std::error::Error;
 use std::fmt;
 use std::io;
 use std::marker::PhantomData;
-use std::ops::{Add, Sub};
+use std::ops::{Add, MulAssign, Sub};
 
 /// Computations are expressed in terms of arithmetic circuits, in particular
 /// rank-1 quadratic constraint systems. The `Circuit` trait represents a