Implement SQL writer

It is often useful to be able to generate SQL from the AST. This opens up possibilities for SQL manipulation or translation. The write should recursively walk the AST and write SQL to a writer / output stream.

A secondary benefit of having a writer is that it often simplifies writing unit tests for the parser, since a SQL statement can be parsed and re-written and then the output can be compared with the input (ignoring whitespace differences)

未指定文件路径或测试路径。首先定位 AST 定义和 parser 入口点，然后检查现有的 parser 单元测试。完成的标准是：通过 writer 或输出流将解析后的 AST 递归写成 SQL，并编写 round-trip 测试，在忽略空白差异的情况下比较解析后的语句和重写后的语句。

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