Eliminating whitespace from the parser logic

Hi,

At this time, whitespace tokens are stored in the parser, and are then filtered out in several distinct points in the parser logic, such as:

* https://github.com/apache/datafusion-sqlparser-rs/blob/67684c84d4c2589356c411ea4917dcf1defcd77c/src/parser/mod.rs#L4032-L4049
* https://github.com/apache/datafusion-sqlparser-rs/blob/67684c84d4c2589356c411ea4917dcf1defcd77c/src/parser/mod.rs#L4055-L4069
* https://github.com/apache/datafusion-sqlparser-rs/blob/67684c84d4c2589356c411ea4917dcf1defcd77c/src/parser/mod.rs#L4077-L4094
* https://github.com/apache/datafusion-sqlparser-rs/blob/67684c84d4c2589356c411ea4917dcf1defcd77c/src/parser/mod.rs#L4149-L4160
* https://github.com/apache/datafusion-sqlparser-rs/blob/67684c84d4c2589356c411ea4917dcf1defcd77c/src/parser/mod.rs#L4183-L4202

and many more.

SQL, as far as I know, is not a language that cares about spaces like Python - it should be safe to remove all concepts of whitespaces after [the tokenization process](https://github.com/apache/datafusion-sqlparser-rs/blob/67684c84d4c2589356c411ea4917dcf1defcd77c/src/tokenizer.rs#L937-L942) is complete, and this should:

* Reduce memory requirements, as whitespace tokens would not be stored anymore
* Significantly simplify parser logic by removing all of the whitespace-related logic from the parser
* Move the parser closer to a streaming logic, but that will require many more PRs

Since such a PR would require quite a bit of effort on my part, I would appreciate some feedback on it before moving forward with it.

@iffyio do you happen to have any opinion regarding such a refactoring?

Ciao,
Luca

首先阅读 src/tokenizer.rs 中链接的 tokenizer 部分，以及 src/parser/mod.rs 中处理空白的示例。跟踪空白 token 如何从 tokenization 流向 parser filtering，然后查看相关的 parser 测试（如果找到）。当提议的空白处理重构得到一致定义，并且 parser 行为仍有测试覆盖时，即视为完成。

由索引模型根据 Issue 内容生成。