Add #[proc_macro_derive(Absorb)] on custom structs

[mmagician]

So that we can write:

#[dervice(Absorb)]
struct MyStruct {
    a: u8,
    b: u8,
}

What do you think? Happy to take it on

[Pratyush]

That would be great!

Would also be great for traits in r1cs-std, like AllocVar, R1CSVar, EqGadget, CmpGadget, etc.