`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`.

先阅读 parse_order_by_expr 的 parser 入口点和现有的 ORDER BY 语法，然后比较相关 parser API 如何处理 ParserError 和 EOF。范围仍需就 iterator 接口达成一致；完成标准包括一个已接受的 API，以及针对多个表达式、EOF 和解析错误的测试。

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