Improve performance by reducing string copies

@Omega359 [mentioned in discord](https://discord.com/channels/885562378132000778/1344430130252218439/1415763431516278794) a blog post by the Databend folks about how they wrote a replacement of sqlparser-rs for their system

https://www.databend.com/blog/category-engineering/2025-09-10-query-parser/

One thing they cite is the fact that sqlparser-rs often copies strings, even when this could be avoided

If anyone cares, we can probably avoid copying strings, someone just has to care enough to thread the lifetime information through all the tokens (or maybe use an Arc w/ an offset)

@AdamGS mentioned he might be interested in trying this

This could possibly be related to another ticket here:
- https://github.com/apache/datafusion-sqlparser-rs/issues/1558

まずリンク先のDatabendブログ記事と関連するissue #1558を読み、その後lexerとtokenの所有権の経路を調べて、文字列がどこでコピーされているかを特定します。回避可能な文字列コピーを削減し、parserの動作を維持したうえで、プロジェクト既存のテストまたはベンチマークを使ってパフォーマンスへの影響を検証できれば完了です。

索引モデルが issue の本文から書いたものです。