Integers Comparator 
by Pig Fang @g-plane
Implement a type-level integers comparator. We've provided an enum for indicating the comparison result, like this:
- If `a` is greater than `b`, type should be `Comparison.Greater`.
- If `a` and `b` are equal, type should be `Comparison.Equal`.
- If `a` is lower than `b`, type should be `Comparison.Lower`.
**Note that `a` and `b` can be positive integers or negative integers or zero, even one is positive while another one is negative.**
