Macro fed::init_fed

macro_rules! init_fed {
    () => { ... };
    (@deriving:[$($derive_trait:ident),*]) => { ... };
    (@deriving:[$($derive_trait:ident,)*]) => { ... };
}

Setup macro to be called after including the fed crate

Unfortunately necessary since the type federation properties cannot be implemented generically, and since the client crate needs to own them in order to be able to implement them for external types.