Macro define_service 

macro_rules! define_service {
    ( $vis:vis $serv:ident : { $($ty:tt)* } ) => { ... };
}

Define a service reference type.

define_service!(MyService : { "f": func!(() -> () query) }) expands to struct MyService(Service), which implements CandidType with the provided type and MyService::new(principal).