Add derive macros

[Sufflope]

Hello,

here is my take on #43 😃 I worked on AbsDiffEq first, it's still a draft obviously.

[Sufflope]

I added RelativeEq support, they mutualize nicely so UlpsEq should be pretty straightforward. I also added support for generics, but TBH I don't really know where to go from there, I miss examples of real use-cases I guess. Should I add a "shortcut" to specify one generic type to use as the backing type, something like

#[derive(AbsDiffEq)]
#[approx(generic = T)]
struct Foo<T> {
  value: T,
  ...
}

or keep it like I did and allow to not necessarily reuse T's characteristics, but be more verbose if you do?

[jonaspleyer]

For anyone looking out for this topic: It seems that development of this crate is stalled. I have written my own lib approx-derive to solve the problem of non-existing derive macros.