Macro define_api_function 

macro_rules! define_api_function {
    (
		$fn_name:ident,
		$closure_type:ident<T, F>,
		$context_type:ident<T>
	) => { ... };
    (
		$fn_name:ident,
		$closure_type:ident<F>,
		$context_type:ident
	) => { ... };
}

Macro to create helper functions that create closure interceptors