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.

该 issue 提到了一个 proc-macro TokenStream 以及 parser 当前的 display-to-raw-string 路径，但没有提供任何 repository 文件或测试。首先定位此输入转换，并确定如何通过直接支持 TokenStream 来保留 tokenization；当能够在不经过 display 和重新 tokenization 步骤的情况下解析 proc-macro TokenStream 时，即表示完成。

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