feat(tvix/eval): implement unary negation operator

Change-Id: I5d012cc073e55d79d7b34b88283aab3164864293
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6075
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/vm.rs

@@ -81,6 +81,18 @@ impl VM {
                     NumberPair::Integer(i1, i2) => self.push(Value::Integer(i1 / i2)),
                 },
 
+                OpCode::OpNegate => match self.pop() {
+                    Value::Integer(i) => self.push(Value::Integer(-i)),
+                    Value::Float(f) => self.push(Value::Float(-f)),
+                    v => {
+                        return Err(Error::TypeError {
+                            expected: "number (either int or float)",
+                            actual: v.type_of(),
+                        })
+                    }
+                },
+
+                OpCode::OpInvert => todo!(),
                 OpCode::OpNull => todo!(),
                 OpCode::OpTrue => todo!(),
                 OpCode::OpFalse => todo!(),