Add support for sum expressions in evaluate()

lib/evaluator.c

@@ -4,5 +4,7 @@ int evaluate(const expression_t *expression)
 {
 	if (expression->is_number)
 		return expression->number;
-	return 0;
+	const int x = evaluate(expression->application.operands[0]);
+	const int y = evaluate(expression->application.operands[1]);
+	return x + y;
 }