Records with (comparable) positional fields should implement Comparable

The following both should work as String and int can be compared to each other.
````dart
print(("a", 1) > ("a", 0)); // false
print(("a", 1).compareTo(("a", 0))); // 1
````

Records should be compared by comparing the fields from left to right until one value isn't equal to the other.

```dart
int compare<
A extends Comparable,
B extends Comparable,
X,
Y
>(A a, B b, X x, Y y) {
return (a, b).compareTo((x, y));
}
```

should return the same as

```dart
int compare<
A extends Comparable,
B extends Comparable,
X,
Y
>(A a, B b, X x, Y y) {
final axCompare = a.compareTo(x);
if (axCompare != 0) {
return axCompare;
}
return b.compareTo(y);
}
```

This would be really useful for sorting a list of records.

```dart
final list = [("a", 1), ("b", 1), ("a", 0), ("b", -1)];
list.sort();
print(list); // (a, 0), (a, 1), (b, -1), (b, 1)
```