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

Beginne mit dem Lesen des verlinkten Tokenizer-Abschnitts in src/tokenizer.rs und der Beispiele zur Behandlung von Whitespace in src/parser/mod.rs. Verfolge, wie Whitespace-Tokens von der Tokenisierung zur Parser-Filterung gelangen, und prüfe anschließend die zugehörigen Parser-Tests, falls vorhanden. Als abgeschlossen gilt die Aufgabe, wenn der vorgeschlagene Refactor zur Behandlung von Whitespace konsistent definiert ist und sein Parser-Verhalten weiterhin abgedeckt wird.

Vom Indexierungsmodell aus dem Issue-Text verfasst.