fn next_token(&mut self) -> Option<Spanned<Token>> {
        if self.index < self.tokens.item.len() {
            let token = &self.tokens.item[self.index];
            self.index += 1;
            // We can clone here because it's not that expensive
            Some(token.clone())
        } else {
            None
        }
    }
Returns the next token, advancing the lexer by 1.
passerine/src/compiler/read.rs