Support `proc-macro` TokenStream

I'm currently writing a proc macro that uses SQL to generate code that works with the storage pattern in [`specs`](https://github.com/slide-rs/specs). As is, I have to call `display` on the `TokenStream`, which converts it to a raw string. This will mean I'm undoing the tokenization for this parser to re-tokenize, while it should be possible for this parser to use the `TokenStream` directly.

This is low priority for me, as this issue only causes excess compilation time, but after creating this proc macro, I can add it to my own to-do.

該 issue 提到了一个 proc-macro TokenStream 以及 parser 目前的 display-to-raw-string 路徑，但沒有提供任何 repository 檔案或測試。首先定位此輸入轉換，並確定如何透過直接支援 TokenStream 來保留 tokenization；當能夠在不經過 display 和重新 tokenization 步驟的情況下解析 proc-macro TokenStream 時，即表示完成。

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