Macro extendr_api::append_lang

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

Convert a list of tokens to an array of tuples.