Macro define_function

macro_rules! define_function {
    ( $vis:vis $func:ident : $($ty:tt)+ ) => { ... };
}

Define a function reference type.

define_function!(pub MyFunc : (u8, &str) -> (Nat) query) expands to pub struct MyFunc(Func), which implements CandidType with the provided type. We also provide a constructor function MyFunc::new(principal, method).