`ORDER BY` parser

Would a specific `ORDER BY` parser fit into this crate? It would generate an iterator which yields `Result`. I have developed a rudimentary version of this, which I am using in production. I could further polish it and open a PR if this would be of interest. I am just repeatedly calling `parse_order_by_expr` and looking for `EOF`.

先閱讀 parse_order_by_expr 的 parser 進入點與現有的 ORDER BY 文法，然後比較相關 parser API 如何處理 ParserError 和 EOF。範圍仍需就 iterator 介面達成共識；完成條件包括一個已接受的 API，以及針對多個運算式、EOF 和解析錯誤的測試。

由索引模型根據 Issue 內容生成。