implement AbsDiffEq for i128

Author: jonaspleyer

approx/src/abs_diff_eq.rs

@@ -110,6 +110,7 @@ impl_signed_abs_diff_eq!(i8, 0);
 impl_signed_abs_diff_eq!(i16, 0);
 impl_signed_abs_diff_eq!(i32, 0);
 impl_signed_abs_diff_eq!(i64, 0);
+impl_signed_abs_diff_eq!(i128, 0);
 impl_signed_abs_diff_eq!(isize, 0);
 impl_signed_abs_diff_eq!(f32, core::f32::EPSILON);
 impl_signed_abs_diff_eq!(f64, core::f64::EPSILON);