Function handle_user_function_call 

pub async fn handle_user_function_call<BF, BFFut, IF, IFFut>(
    ctx: UserCallContext<'_>,
    arg_count: usize,
    refresh_vars: impl Fn(&[Value]),
    builtin_fallback: BF,
    interpret_counts: IF,
) -> Result<UserCallHandling, RuntimeError>
where
    BF: FnOnce(String, Vec<Value>, usize) -> BFFut,
    BFFut: Future<Output = Result<Option<Value>, RuntimeError>>,
    IF: FnOnce(Bytecode, Vec<Value>, String, usize, usize) -> IFFut,
    IFFut: Future<Output = Result<Vec<Value>, RuntimeError>>,