Support all operations for Value<T>.

[arseniybelkov]

All the operations are implemented only for &Value<T>, which forbids the following code:

let x = Value::new(2f32);
let y = Value::new(3f32);
let z = &x + y; // Add is not implemented ...
z.backward();

In the case above, y should probably be stored in the graph, to make the backprop possible through it. It might be tempting to add indirection level via Box or Rc, but it must be done using no heap allocations.