Bounds: Make `Comparable` require only `__lt__`

To evaluate if a value is within bounds, we only need to the underlying type to support `__lt__`. This is also what Python built-ins do ([`list.sort`](https://docs.python.org/3/library/stdtypes.html#list.sort) and [`sorted`](https://docs.python.org/3/library/functions.html#sorted)), so it will be more intuitive to users.