`std::abs()` doesn't work on custom types. While one can use the 3-argument version of `DistanceFrom()` to specify how to compute the distance, it's not as convenient as defining `abs()` for the custom type once in the type's namespace and then use the 2-argument version.
PiperOrigin-RevId: 735741409
Change-Id: If8fb668455eb963a2ccf089f7467c64965a2e7a6