Handle `cmp::Reverse` keys

[TheIronBorn]

std::cmp::Reverse is very useful for lexicographic tuples. i.e.

radsort::sort_by_key(list, |x| (x.0, Reverse(x.1)));

Signed types can just be negated but unsigned types cannot and currently would need a cumbersome sort_by function or a risky conversion to a signed type.

[JakubValtar]

Thanks for the suggestion, I will see if I can make reversing the order more convenient.

For now, you can use these operators:

radsort::sort_by_key(list, |x| !x); // unsigned integers, signed integers, bool
radsort::sort_by_key(list, |x| -x); // f32, f64, signed integers (though `i_::MIN` will cause an overflow)
radsort::sort_by_key(list, |x| !(x as u32)); // char

[TheIronBorn]

I wasn't aware bitwise NOT worked for this. Thank you