Oh man.

#cplusplus supports a user-defined literal. So if you define an `MyDistanceType operator""_km(long double val)` function, it'll allow you to say `const auto distance = 12_km;` and cause that to evaluate to a MyDistanceType representing 12 kilometers.

Cute, but I really can't see why anyone would bother when the language already supports `const auto distance = Distance::from_kilometers(12);`