Consider providing a derive macro

[Kixunil]

It'd be helpful to have a macro deriving ArbitraryOrd, just like derive(Ord). I have also realized there are hacks that allow us to emit code that is specialized - can use Ord or ArbitraryOrd depending on which is implemented and picks one specific of them if both are. (Note that this cannot be done with generics, it needs deref trick or i32 fallback.)

This is not required for 1.0 but it could be very helpful for some crates.