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.