Macro extendr_api::push_args

macro_rules! push_args {
    ($args: expr, $name: ident = $val : expr) => { ... };
    ($args: expr, $name: ident = $val : expr, $($rest: tt)*) => { ... };
    ($args: expr, $val : expr) => { ... };
    ($args: expr, $val : expr, $($rest: tt)*) => { ... };
}

Convert a list of tokens to an array of tuples.