Filter some macros

[hustxiaoc]

For some macros like log!, trace! is there a way to filter them?

[Mingun]

Even better if you could specify what macros should keep unexpanded. For example, when examining output of the serde_derive generated code I would be like to not expand try! macro.

[elmarco]

I think #68 would be a more convenient solution (not filtering out, but rather specifying what you want expanded).

[dtolnay]

Closing in favor of #68 since these are about effectively equivalent functionality.

[Mingun]

@dtolnay , just do not forget about case with serde_derive: try! macro defined in the serde_derive generated code and I'd like to not expand it