Add a couple operators.

src/op.rs

@@ -2,13 +2,17 @@ use crate::Tensor;
 
 pub(crate) enum Op {
     Add(Tensor, Tensor),
+    Mul(Tensor, Tensor),
+    Sub(Tensor, Tensor),
+    Div(Tensor, Tensor),
+
     #[allow(dead_code)] // add is currently unused.
     Affine {
         arg: Tensor,
         mul: f64,
         add: f64,
     },
-    Mul(Tensor, Tensor),
+    Neg(Tensor),
     Sqr(Tensor),
     Sqrt(Tensor),
     // TODO: Support for custom ops.