Move from Field::negate to Neg operator

ff/src/lib.rs

@@ -11,7 +11,7 @@ use rand_core::RngCore;
 use std::error::Error;
 use std::fmt;
 use std::io::{self, Read, Write};
-use std::ops::{Add, AddAssign, Mul, MulAssign, Sub, SubAssign};
+use std::ops::{Add, AddAssign, Mul, MulAssign, Neg, Sub, SubAssign};
 
 /// This trait represents an element of a field.
 pub trait Field:
@@ -27,6 +27,7 @@ pub trait Field:
     + Add<Output = Self>
     + Sub<Output = Self>
     + Mul<Output = Self>
+    + Neg<Output = Self>
     + for<'a> Add<&'a Self, Output = Self>
     + for<'a> Mul<&'a Self, Output = Self>
     + for<'a> Sub<&'a Self, Output = Self>
@@ -55,9 +56,6 @@ pub trait Field:
     /// Doubles this element.
     fn double(&mut self);
 
-    /// Negates this element.
-    fn negate(&mut self);
-
     /// Computes the multiplicative inverse of this element, if nonzero.
     fn inverse(&self) -> Option<Self>;